Controlling Application's Volume: By Process-ID

Here is a set of utility C# classes that allow you to get information on audio devices and audio session, including process information. They use the Windows Core Audio Library, and only work on Windows 7 or higher:

// sample program
class Program
{
    static void Main(string[] args)
    {
        // dump all audio devices
        foreach (AudioDevice device in AudioUtilities.GetAllDevices())
        {
            Console.WriteLine(device.FriendlyName);
        }

        // dump all audio sessions
        foreach (AudioSession session in AudioUtilities.GetAllSessions())
        {
            if (session.Process != null)
            {
                // only the one associated with a defined process
                Console.WriteLine(session.Process.ProcessName);
            }
        }
    }
}

// audio utilities
public static class AudioUtilities
{
    private static IAudioSessionManager2 GetAudioSessionManager()
    {
        IMMDevice speakers = GetSpeakers();
        if (speakers == null)
            return null;

        // win7+ only
        object o;
        if (speakers.Activate(typeof(IAudioSessionManager2).GUID, CLSCTX.CLSCTX_ALL, IntPtr.Zero, out o) != 0 || o == null)
            return null;

        return o as IAudioSessionManager2;
    }

    public static AudioDevice GetSpeakersDevice()
    {
        return CreateDevice(GetSpeakers());
    }

    private static AudioDevice CreateDevice(IMMDevice dev)
    {
        if (dev == null)
            return null;

        string id;
        dev.GetId(out id);
        DEVICE_STATE state;
        dev.GetState(out state);
        Dictionary<string, object> properties = new Dictionary<string, object>();
        IPropertyStore store;
        dev.OpenPropertyStore(STGM.STGM_READ, out store);
        if (store != null)
        {
            int propCount;
            store.GetCount(out propCount);
            for (int j = 0; j < propCount; j++)
            {
                PROPERTYKEY pk;
                if (store.GetAt(j, out pk) == 0)
                {
                    PROPVARIANT value = new PROPVARIANT();
                    int hr = store.GetValue(ref pk, ref value);
                    object v = value.GetValue();
                    try
                    {
                        if (value.vt != VARTYPE.VT_BLOB) // for some reason, this fails?
                        {
                            PropVariantClear(ref value);
                        }
                    }
                    catch
                    {
                    }
                    string name = pk.ToString();
                    properties[name] = v;
                }
            }
        }
        return new AudioDevice(id, (AudioDeviceState)state, properties);
    }

    public static IList<AudioDevice> GetAllDevices()
    {
        List<AudioDevice> list = new List<AudioDevice>();
        IMMDeviceEnumerator deviceEnumerator = null;
        try
        {
            deviceEnumerator = (IMMDeviceEnumerator)(new MMDeviceEnumerator());
        }
        catch
        {
        }
        if (deviceEnumerator == null)
            return list;

        IMMDeviceCollection collection;
        deviceEnumerator.EnumAudioEndpoints(EDataFlow.eAll, DEVICE_STATE.MASK_ALL, out collection);
        if (collection == null)
            return list;

        int count;
        collection.GetCount(out count);
        for (int i = 0; i < count; i++)
        {
            IMMDevice dev;
            collection.Item(i, out dev);
            if (dev != null)
            {
                list.Add(CreateDevice(dev));
            }
        }
        return list;
    }

    private static IMMDevice GetSpeakers()
    {
        // get the speakers (1st render + multimedia) device
        try
        {
            IMMDeviceEnumerator deviceEnumerator = (IMMDeviceEnumerator)(new MMDeviceEnumerator());
            IMMDevice speakers;
            deviceEnumerator.GetDefaultAudioEndpoint(EDataFlow.eRender, ERole.eMultimedia, out speakers);
            return speakers;
        }
        catch
        {
            return null;
        }
    }

    public static IList<AudioSession> GetAllSessions()
    {
        List<AudioSession> list = new List<AudioSession>();
        IAudioSessionManager2 mgr = GetAudioSessionManager();
        if (mgr == null)
            return list;

        IAudioSessionEnumerator sessionEnumerator;
        mgr.GetSessionEnumerator(out sessionEnumerator);
        int count;
        sessionEnumerator.GetCount(out count);

        for (int i = 0; i < count; i++)
        {
            IAudioSessionControl ctl;
            sessionEnumerator.GetSession(i, out ctl);
            if (ctl == null)
                continue;

            IAudioSessionControl2 ctl2 = ctl as IAudioSessionControl2;
            if (ctl2 != null)
            {
                list.Add(new AudioSession(ctl2));
            }
        }
        Marshal.ReleaseComObject(sessionEnumerator);
        Marshal.ReleaseComObject(mgr);
        return list;
    }

    public static AudioSession GetProcessSession()
    {
        int id = Process.GetCurrentProcess().Id;
        foreach (AudioSession session in GetAllSessions())
        {
            if (session.ProcessId == id)
                return session;

            session.Dispose();
        }
        return null;
    }

    [DllImport("ole32.dll")]
    private static extern int PropVariantClear(ref PROPVARIANT pvar);

    [ComImport]
    [Guid("BCDE0395-E52F-467C-8E3D-C4579291692E")]
    private class MMDeviceEnumerator
    {
    }

    [Flags]
    private enum CLSCTX
    {
        CLSCTX_INPROC_SERVER = 0x1,
        CLSCTX_INPROC_HANDLER = 0x2,
        CLSCTX_LOCAL_SERVER = 0x4,
        CLSCTX_REMOTE_SERVER = 0x10,
        CLSCTX_ALL = CLSCTX_INPROC_SERVER | CLSCTX_INPROC_HANDLER | CLSCTX_LOCAL_SERVER | CLSCTX_REMOTE_SERVER
    }

    private enum STGM
    {
        STGM_READ = 0x00000000,
    }

    private enum EDataFlow
    {
        eRender,
        eCapture,
        eAll,
    }

    private enum ERole
    {
        eConsole,
        eMultimedia,
        eCommunications,
    }

    private enum DEVICE_STATE
    {
        ACTIVE = 0x00000001,
        DISABLED = 0x00000002,
        NOTPRESENT = 0x00000004,
        UNPLUGGED = 0x00000008,
        MASK_ALL = 0x0000000F
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct PROPERTYKEY
    {
        public Guid fmtid;
        public int pid;

        public override string ToString()
        {
            return fmtid.ToString("B") + " " + pid;
        }
    }

    // NOTE: we only define what we handle
    [Flags]
    private enum VARTYPE : short
    {
        VT_I4 = 3,
        VT_BOOL = 11,
        VT_UI4 = 19,
        VT_LPWSTR = 31,
        VT_BLOB = 65,
        VT_CLSID = 72,
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct PROPVARIANT
    {
        public VARTYPE vt;
        public ushort wReserved1;
        public ushort wReserved2;
        public ushort wReserved3;
        public PROPVARIANTunion union;

        public object GetValue()
        {
            switch (vt)
            {
                case VARTYPE.VT_BOOL:
                    return union.boolVal != 0;

                case VARTYPE.VT_LPWSTR:
                    return Marshal.PtrToStringUni(union.pwszVal);

                case VARTYPE.VT_UI4:
                    return union.lVal;

                case VARTYPE.VT_CLSID:
                    return (Guid)Marshal.PtrToStructure(union.puuid, typeof(Guid));

                default:
                    return vt.ToString() + ":?";
            }
        }
    }

    [StructLayout(LayoutKind.Explicit)]
    private struct PROPVARIANTunion
    {
        [FieldOffset(0)]
        public int lVal;
        [FieldOffset(0)]
        public ulong uhVal;
        [FieldOffset(0)]
        public short boolVal;
        [FieldOffset(0)]
        public IntPtr pwszVal;
        [FieldOffset(0)]
        public IntPtr puuid;
    }

    [Guid("A95664D2-9614-4F35-A746-DE8DB63617E6"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IMMDeviceEnumerator
    {
        [PreserveSig]
        int EnumAudioEndpoints(EDataFlow dataFlow, DEVICE_STATE dwStateMask, out IMMDeviceCollection ppDevices);

        [PreserveSig]
        int GetDefaultAudioEndpoint(EDataFlow dataFlow, ERole role, out IMMDevice ppEndpoint);

        [PreserveSig]
        int GetDevice([MarshalAs(UnmanagedType.LPWStr)] string pwstrId, out IMMDevice ppDevice);

        [PreserveSig]
        int RegisterEndpointNotificationCallback(IMMNotificationClient pClient);

        [PreserveSig]
        int UnregisterEndpointNotificationCallback(IMMNotificationClient pClient);
    }

    [Guid("7991EEC9-7E89-4D85-8390-6C703CEC60C0"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IMMNotificationClient
    {
        void OnDeviceStateChanged([MarshalAs(UnmanagedType.LPWStr)] string pwstrDeviceId, DEVICE_STATE dwNewState);
        void OnDeviceAdded([MarshalAs(UnmanagedType.LPWStr)] string pwstrDeviceId);
        void OnDeviceRemoved([MarshalAs(UnmanagedType.LPWStr)] string deviceId);
        void OnDefaultDeviceChanged(EDataFlow flow, ERole role, string pwstrDefaultDeviceId);
        void OnPropertyValueChanged([MarshalAs(UnmanagedType.LPWStr)] string pwstrDeviceId, PROPERTYKEY key);
    }

    [Guid("0BD7A1BE-7A1A-44DB-8397-CC5392387B5E"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IMMDeviceCollection
    {
        [PreserveSig]
        int GetCount(out int pcDevices);

        [PreserveSig]
        int Item(int nDevice, out IMMDevice ppDevice);
    }

    [Guid("D666063F-1587-4E43-81F1-B948E807363F"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IMMDevice
    {
        [PreserveSig]
        int Activate([MarshalAs(UnmanagedType.LPStruct)] Guid riid, CLSCTX dwClsCtx, IntPtr pActivationParams, [MarshalAs(UnmanagedType.IUnknown)] out object ppInterface);

        [PreserveSig]
        int OpenPropertyStore(STGM stgmAccess, out IPropertyStore ppProperties);

        [PreserveSig]
        int GetId([MarshalAs(UnmanagedType.LPWStr)] out string ppstrId);

        [PreserveSig]
        int GetState(out DEVICE_STATE pdwState);
    }

    [Guid("6f79d558-3e96-4549-a1d1-7d75d2288814"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IPropertyDescription
    {
        [PreserveSig]
        int GetPropertyKey(out PROPERTYKEY pkey);

        [PreserveSig]
        int GetCanonicalName(out IntPtr ppszName);

        [PreserveSig]
        int GetPropertyType(out short pvartype);

        [PreserveSig]
        int GetDisplayName(out IntPtr ppszName);

        // WARNING: the rest is undefined. you *can't* implement it, only use it.
    }

    [Guid("886d8eeb-8cf2-4446-8d02-cdba1dbdcf99"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IPropertyStore
    {
        [PreserveSig]
        int GetCount(out int cProps);

        [PreserveSig]
        int GetAt(int iProp, out PROPERTYKEY pkey);

        [PreserveSig]
        int GetValue(ref PROPERTYKEY key, ref PROPVARIANT pv);

        [PreserveSig]
        int SetValue(ref PROPERTYKEY key, ref PROPVARIANT propvar);

        [PreserveSig]
        int Commit();
    }

    [Guid("BFA971F1-4D5E-40BB-935E-967039BFBEE4"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IAudioSessionManager
    {
        [PreserveSig]
        int GetAudioSessionControl([MarshalAs(UnmanagedType.LPStruct)] Guid AudioSessionGuid, int StreamFlags, out IAudioSessionControl SessionControl);

        [PreserveSig]
        int GetSimpleAudioVolume([MarshalAs(UnmanagedType.LPStruct)] Guid AudioSessionGuid, int StreamFlags, ISimpleAudioVolume AudioVolume);
    }

    [Guid("77AA99A0-1BD6-484F-8BC7-2C654C9A9B6F"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IAudioSessionManager2
    {
        [PreserveSig]
        int GetAudioSessionControl([MarshalAs(UnmanagedType.LPStruct)] Guid AudioSessionGuid, int StreamFlags, out IAudioSessionControl SessionControl);

        [PreserveSig]
        int GetSimpleAudioVolume([MarshalAs(UnmanagedType.LPStruct)] Guid AudioSessionGuid, int StreamFlags, ISimpleAudioVolume AudioVolume);

        [PreserveSig]
        int GetSessionEnumerator(out IAudioSessionEnumerator SessionEnum);

        [PreserveSig]
        int RegisterSessionNotification(IAudioSessionNotification SessionNotification);

        [PreserveSig]
        int UnregisterSessionNotification(IAudioSessionNotification SessionNotification);

        int RegisterDuckNotificationNotImpl();
        int UnregisterDuckNotificationNotImpl();
    }

    [Guid("641DD20B-4D41-49CC-ABA3-174B9477BB08"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IAudioSessionNotification
    {
        void OnSessionCreated(IAudioSessionControl NewSession);
    }

    [Guid("E2F5BB11-0570-40CA-ACDD-3AA01277DEE8"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IAudioSessionEnumerator
    {
        [PreserveSig]
        int GetCount(out int SessionCount);

        [PreserveSig]
        int GetSession(int SessionCount, out IAudioSessionControl Session);
    }

    [Guid("bfb7ff88-7239-4fc9-8fa2-07c950be9c6d"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    internal interface IAudioSessionControl2
    {
        // IAudioSessionControl
        [PreserveSig]
        int GetState(out AudioSessionState pRetVal);

        [PreserveSig]
        int GetDisplayName([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

        [PreserveSig]
        int SetDisplayName([MarshalAs(UnmanagedType.LPWStr)]string Value, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

        [PreserveSig]
        int GetIconPath([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

        [PreserveSig]
        int SetIconPath([MarshalAs(UnmanagedType.LPWStr)] string Value, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

        [PreserveSig]
        int GetGroupingParam(out Guid pRetVal);

        [PreserveSig]
        int SetGroupingParam([MarshalAs(UnmanagedType.LPStruct)] Guid Override, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

        [PreserveSig]
        int RegisterAudioSessionNotification(IAudioSessionEvents NewNotifications);

        [PreserveSig]
        int UnregisterAudioSessionNotification(IAudioSessionEvents NewNotifications);

        // IAudioSessionControl2
        [PreserveSig]
        int GetSessionIdentifier([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

        [PreserveSig]
        int GetSessionInstanceIdentifier([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

        [PreserveSig]
        int GetProcessId(out int pRetVal);

        [PreserveSig]
        int IsSystemSoundsSession();

        [PreserveSig]
        int SetDuckingPreference(bool optOut);
    }

    [Guid("F4B1A599-7266-4319-A8CA-E70ACB11E8CD"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    internal interface IAudioSessionControl
    {
        [PreserveSig]
        int GetState(out AudioSessionState pRetVal);

        [PreserveSig]
        int GetDisplayName([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

        [PreserveSig]
        int SetDisplayName([MarshalAs(UnmanagedType.LPWStr)]string Value, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

        [PreserveSig]
        int GetIconPath([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

        [PreserveSig]
        int SetIconPath([MarshalAs(UnmanagedType.LPWStr)] string Value, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

        [PreserveSig]
        int GetGroupingParam(out Guid pRetVal);

        [PreserveSig]
        int SetGroupingParam([MarshalAs(UnmanagedType.LPStruct)] Guid Override, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

        [PreserveSig]
        int RegisterAudioSessionNotification(IAudioSessionEvents NewNotifications);

        [PreserveSig]
        int UnregisterAudioSessionNotification(IAudioSessionEvents NewNotifications);
    }

    [Guid("24918ACC-64B3-37C1-8CA9-74A66E9957A8"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    internal interface IAudioSessionEvents
    {
        void OnDisplayNameChanged([MarshalAs(UnmanagedType.LPWStr)] string NewDisplayName, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);
        void OnIconPathChanged([MarshalAs(UnmanagedType.LPWStr)] string NewIconPath, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);
        void OnSimpleVolumeChanged(float NewVolume, bool NewMute, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);
        void OnChannelVolumeChanged(int ChannelCount, IntPtr NewChannelVolumeArray, int ChangedChannel, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);
        void OnGroupingParamChanged([MarshalAs(UnmanagedType.LPStruct)] Guid NewGroupingParam, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);
        void OnStateChanged(AudioSessionState NewState);
        void OnSessionDisconnected(AudioSessionDisconnectReason DisconnectReason);
    }

    [Guid("87CE5498-68D6-44E5-9215-6DA47EF883D8"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface ISimpleAudioVolume
    {
        [PreserveSig]
        int SetMasterVolume(float fLevel, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

        [PreserveSig]
        int GetMasterVolume(out float pfLevel);

        [PreserveSig]
        int SetMute(bool bMute, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

        [PreserveSig]
        int GetMute(out bool pbMute);
    }
}

public sealed class AudioSession : IDisposable
{
    private AudioUtilities.IAudioSessionControl2 _ctl;
    private Process _process;

    internal AudioSession(AudioUtilities.IAudioSessionControl2 ctl)
    {
        _ctl = ctl;
    }

    public Process Process
    {
        get
        {
            if (_process == null && ProcessId != 0)
            {
                try
                {
                    _process = Process.GetProcessById(ProcessId);
                }
                catch
                {
                    // do nothing
                }
            }
            return _process;
        }
    }

    public int ProcessId
    {
        get
        {
            CheckDisposed();
            int i;
            _ctl.GetProcessId(out i);
            return i;
        }
    }

    public string Identifier
    {
        get
        {
            CheckDisposed();
            string s;
            _ctl.GetSessionIdentifier(out s);
            return s;
        }
    }

    public string InstanceIdentifier
    {
        get
        {
            CheckDisposed();
            string s;
            _ctl.GetSessionInstanceIdentifier(out s);
            return s;
        }
    }

    public AudioSessionState State
    {
        get
        {
            CheckDisposed();
            AudioSessionState s;
            _ctl.GetState(out s);
            return s;
        }
    }

    public Guid GroupingParam
    {
        get
        {
            CheckDisposed();
            Guid g;
            _ctl.GetGroupingParam(out g);
            return g;
        }
        set
        {
            CheckDisposed();
            _ctl.SetGroupingParam(value, Guid.Empty);
        }
    }

    public string DisplayName
    {
        get
        {
            CheckDisposed();
            string s;
            _ctl.GetDisplayName(out s);
            return s;
        }
        set
        {
            CheckDisposed();
            string s;
            _ctl.GetDisplayName(out s);
            if (s != value)
            {
                _ctl.SetDisplayName(value, Guid.Empty);
            }
        }
    }

    public string IconPath
    {
        get
        {
            CheckDisposed();
            string s;
            _ctl.GetIconPath(out s);
            return s;
        }
        set
        {
            CheckDisposed();
            string s;
            _ctl.GetIconPath(out s);
            if (s != value)
            {
                _ctl.SetIconPath(value, Guid.Empty);
            }
        }
    }

    private void CheckDisposed()
    {
        if (_ctl == null)
            throw new ObjectDisposedException("Control");
    }

    public override string ToString()
    {
        string s = DisplayName;
        if (!string.IsNullOrEmpty(s))
            return "DisplayName: " + s;

        if (Process != null)
            return "Process: " + Process.ProcessName;

        return "Pid: " + ProcessId;
    }

    public void Dispose()
    {
        if (_ctl != null)
        {
            Marshal.ReleaseComObject(_ctl);
            _ctl = null;
        }
    }
}

public sealed class AudioDevice
{
    internal AudioDevice(string id, AudioDeviceState state, IDictionary<string, object> properties)
    {
        Id = id;
        State = state;
        Properties = properties;
    }

    public string Id { get; private set; }
    public AudioDeviceState State { get; private set; }
    public IDictionary<string, object> Properties { get; private set; }

    public string Description
    {
        get
        {
            const string PKEY_Device_DeviceDesc = "{a45c254e-df1c-4efd-8020-67d146a850e0} 2";
            object value;
            Properties.TryGetValue(PKEY_Device_DeviceDesc, out value);
            return string.Format("{0}", value);
        }
    }

    public string ContainerId
    {
        get
        {
            const string PKEY_Devices_ContainerId = "{8c7ed206-3f8a-4827-b3ab-ae9e1faefc6c} 2";
            object value;
            Properties.TryGetValue(PKEY_Devices_ContainerId, out value);
            return string.Format("{0}", value);
        }
    }

    public string EnumeratorName
    {
        get
        {
            const string PKEY_Device_EnumeratorName = "{a45c254e-df1c-4efd-8020-67d146a850e0} 24";
            object value;
            Properties.TryGetValue(PKEY_Device_EnumeratorName, out value);
            return string.Format("{0}", value);
        }
    }

    public string InterfaceFriendlyName
    {
        get
        {
            const string DEVPKEY_DeviceInterface_FriendlyName = "{026e516e-b814-414b-83cd-856d6fef4822} 2";
            object value;
            Properties.TryGetValue(DEVPKEY_DeviceInterface_FriendlyName, out value);
            return string.Format("{0}", value);
        }
    }

    public string FriendlyName
    {
        get
        {
            const string DEVPKEY_Device_FriendlyName = "{a45c254e-df1c-4efd-8020-67d146a850e0} 14";
            object value;
            Properties.TryGetValue(DEVPKEY_Device_FriendlyName, out value);
            return string.Format("{0}", value);
        }
    }

    public string IconPath
    {
        get
        {
            const string DEVPKEY_DeviceClass_IconPath = "{259abffc-50a7-47ce-af08-68c9a7d73366} 12";
            object value;
            Properties.TryGetValue(DEVPKEY_DeviceClass_IconPath, out value);
            return string.Format("{0}", value);
        }
    }

    public override string ToString()
    {
        return FriendlyName;
    }
}

public enum AudioSessionState
{
    Inactive = 0,
    Active = 1,
    Expired = 2
}

public enum AudioDeviceState
{
    Active = 0x1,
    Disabled = 0x2,
    NotPresent = 0x4,
    Unplugged = 0x8,
}

public enum AudioSessionDisconnectReason
{
    DisconnectReasonDeviceRemoval = 0,
    DisconnectReasonServerShutdown = 1,
    DisconnectReasonFormatChanged = 2,
    DisconnectReasonSessionLogoff = 3,
    DisconnectReasonSessionDisconnected = 4,
    DisconnectReasonExclusiveModeOverride = 5
}

I took Simon's answer from this question and the other question, and boiled it down to answer the question: How to change the volume of a program only given its process ID.

This example sets the volume of Spotify to 50%.

static class Program {
  [DllImport("user32.dll")]
  public static extern IntPtr FindWindow(string strClassName, string strWindowName);

  [DllImport("user32.dll", SetLastError = true)]
  public static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint processId);

  static void Main() {
    var pID = 0;
    foreach (var process in Process.GetProcesses()) {
      if (process.ProcessName == "Spotify" && !String.IsNullOrEmpty(process.MainWindowTitle))
        pID = process.Id;
    }

    if (pID == 0)
      return;

    VolumeMixer.SetApplicationVolume(pID, 50f);
  }
}

Here is the VolumeMixer helper class:

public class VolumeMixer {
  public static float? GetApplicationVolume(int pid) {
    ISimpleAudioVolume volume = GetVolumeObject(pid);
    if (volume == null)
      return null;

    float level;
    volume.GetMasterVolume(out level);
    Marshal.ReleaseComObject(volume);
    return level * 100;
  }

  public static bool? GetApplicationMute(int pid) {
    ISimpleAudioVolume volume = GetVolumeObject(pid);
    if (volume == null)
      return null;

    bool mute;
    volume.GetMute(out mute);
    Marshal.ReleaseComObject(volume);
    return mute;
  }

  public static void SetApplicationVolume(int pid, float level) {
    ISimpleAudioVolume volume = GetVolumeObject(pid);
    if (volume == null)
      return;

    Guid guid = Guid.Empty;
    volume.SetMasterVolume(level / 100, ref guid);
    Marshal.ReleaseComObject(volume);
  }

  public static void SetApplicationMute(int pid, bool mute) {
    ISimpleAudioVolume volume = GetVolumeObject(pid);
    if (volume == null)
      return;

    Guid guid = Guid.Empty;
    volume.SetMute(mute, ref guid);
    Marshal.ReleaseComObject(volume);
  }

  private static ISimpleAudioVolume GetVolumeObject(int pid) {
    // get the speakers (1st render + multimedia) device
    IMMDeviceEnumerator deviceEnumerator = (IMMDeviceEnumerator)(new MMDeviceEnumerator());
    IMMDevice speakers;
    deviceEnumerator.GetDefaultAudioEndpoint(EDataFlow.eRender, ERole.eMultimedia, out speakers);

    // activate the session manager. we need the enumerator
    Guid IID_IAudioSessionManager2 = typeof(IAudioSessionManager2).GUID;
    object o;
    speakers.Activate(ref IID_IAudioSessionManager2, 0, IntPtr.Zero, out o);
    IAudioSessionManager2 mgr = (IAudioSessionManager2)o;

    // enumerate sessions for on this device
    IAudioSessionEnumerator sessionEnumerator;
    mgr.GetSessionEnumerator(out sessionEnumerator);
    int count;
    sessionEnumerator.GetCount(out count);

    // search for an audio session with the required name
    // NOTE: we could also use the process id instead of the app name (with IAudioSessionControl2)
    ISimpleAudioVolume volumeControl = null;
    for (int i = 0; i < count; i++) {
      IAudioSessionControl2 ctl;
      sessionEnumerator.GetSession(i, out ctl);
      int cpid;
      ctl.GetProcessId(out cpid);

      if (cpid == pid) {
        volumeControl = ctl as ISimpleAudioVolume;
        break;
      }
      Marshal.ReleaseComObject(ctl);
    }
    Marshal.ReleaseComObject(sessionEnumerator);
    Marshal.ReleaseComObject(mgr);
    Marshal.ReleaseComObject(speakers);
    Marshal.ReleaseComObject(deviceEnumerator);
    return volumeControl;
  }
}

[ComImport]
[Guid("BCDE0395-E52F-467C-8E3D-C4579291692E")]
internal class MMDeviceEnumerator {
}

internal enum EDataFlow {
  eRender,
  eCapture,
  eAll,
  EDataFlow_enum_count
}

internal enum ERole {
  eConsole,
  eMultimedia,
  eCommunications,
  ERole_enum_count
}

[Guid("A95664D2-9614-4F35-A746-DE8DB63617E6"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IMMDeviceEnumerator {
  int NotImpl1();

  [PreserveSig]
  int GetDefaultAudioEndpoint(EDataFlow dataFlow, ERole role, out IMMDevice ppDevice);

  // the rest is not implemented
}

[Guid("D666063F-1587-4E43-81F1-B948E807363F"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IMMDevice {
  [PreserveSig]
  int Activate(ref Guid iid, int dwClsCtx, IntPtr pActivationParams, [MarshalAs(UnmanagedType.IUnknown)] out object ppInterface);

  // the rest is not implemented
}

[Guid("77AA99A0-1BD6-484F-8BC7-2C654C9A9B6F"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IAudioSessionManager2 {
  int NotImpl1();
  int NotImpl2();

  [PreserveSig]
  int GetSessionEnumerator(out IAudioSessionEnumerator SessionEnum);

  // the rest is not implemented
}

[Guid("E2F5BB11-0570-40CA-ACDD-3AA01277DEE8"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IAudioSessionEnumerator {
  [PreserveSig]
  int GetCount(out int SessionCount);

  [PreserveSig]
  int GetSession(int SessionCount, out IAudioSessionControl2 Session);
}

[Guid("87CE5498-68D6-44E5-9215-6DA47EF883D8"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface ISimpleAudioVolume {
  [PreserveSig]
  int SetMasterVolume(float fLevel, ref Guid EventContext);

  [PreserveSig]
  int GetMasterVolume(out float pfLevel);

  [PreserveSig]
  int SetMute(bool bMute, ref Guid EventContext);

  [PreserveSig]
  int GetMute(out bool pbMute);
}

[Guid("bfb7ff88-7239-4fc9-8fa2-07c950be9c6d"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IAudioSessionControl2 {
  // IAudioSessionControl
  [PreserveSig]
  int NotImpl0();

  [PreserveSig]
  int GetDisplayName([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

  [PreserveSig]
  int SetDisplayName([MarshalAs(UnmanagedType.LPWStr)]string Value, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

  [PreserveSig]
  int GetIconPath([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

  [PreserveSig]
  int SetIconPath([MarshalAs(UnmanagedType.LPWStr)] string Value, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

  [PreserveSig]
  int GetGroupingParam(out Guid pRetVal);

  [PreserveSig]
  int SetGroupingParam([MarshalAs(UnmanagedType.LPStruct)] Guid Override, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

  [PreserveSig]
  int NotImpl1();

  [PreserveSig]
  int NotImpl2();

  // IAudioSessionControl2
  [PreserveSig]
  int GetSessionIdentifier([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

  [PreserveSig]
  int GetSessionInstanceIdentifier([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

  [PreserveSig]
  int GetProcessId(out int pRetVal);

  [PreserveSig]
  int IsSystemSoundsSession();

  [PreserveSig]
  int SetDuckingPreference(bool optOut);
}