How can I mute/unmute my sound from PowerShell

Starting with Vista you have to use the Core Audio API to control the system volume. It's a COM API that doesn't support automation and thus requires a lot of boilerplate to use from .NET and PowerShell.

Anyways the code bellow let you access the [Audio]::Volume and [Audio]::Mute properties from PowerShell. This also work on a remote computer which could be useful. Just copy-paste the code in your PowerShell window.

Add-Type -TypeDefinition @'
using System.Runtime.InteropServices;

[Guid("5CDF2C82-841E-4546-9722-0CF74078229A"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
interface IAudioEndpointVolume {
  // f(), g(), ... are unused COM method slots. Define these if you care
  int f(); int g(); int h(); int i();
  int SetMasterVolumeLevelScalar(float fLevel, System.Guid pguidEventContext);
  int j();
  int GetMasterVolumeLevelScalar(out float pfLevel);
  int k(); int l(); int m(); int n();
  int SetMute([MarshalAs(UnmanagedType.Bool)] bool bMute, System.Guid pguidEventContext);
  int GetMute(out bool pbMute);
}
[Guid("D666063F-1587-4E43-81F1-B948E807363F"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
interface IMMDevice {
  int Activate(ref System.Guid id, int clsCtx, int activationParams, out IAudioEndpointVolume aev);
}
[Guid("A95664D2-9614-4F35-A746-DE8DB63617E6"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
interface IMMDeviceEnumerator {
  int f(); // Unused
  int GetDefaultAudioEndpoint(int dataFlow, int role, out IMMDevice endpoint);
}
[ComImport, Guid("BCDE0395-E52F-467C-8E3D-C4579291692E")] class MMDeviceEnumeratorComObject { }

public class Audio {
  static IAudioEndpointVolume Vol() {
    var enumerator = new MMDeviceEnumeratorComObject() as IMMDeviceEnumerator;
    IMMDevice dev = null;
    Marshal.ThrowExceptionForHR(enumerator.GetDefaultAudioEndpoint(/*eRender*/ 0, /*eMultimedia*/ 1, out dev));
    IAudioEndpointVolume epv = null;
    var epvid = typeof(IAudioEndpointVolume).GUID;
    Marshal.ThrowExceptionForHR(dev.Activate(ref epvid, /*CLSCTX_ALL*/ 23, 0, out epv));
    return epv;
  }
  public static float Volume {
    get {float v = -1; Marshal.ThrowExceptionForHR(Vol().GetMasterVolumeLevelScalar(out v)); return v;}
    set {Marshal.ThrowExceptionForHR(Vol().SetMasterVolumeLevelScalar(value, System.Guid.Empty));}
  }
  public static bool Mute {
    get { bool mute; Marshal.ThrowExceptionForHR(Vol().GetMute(out mute)); return mute; }
    set { Marshal.ThrowExceptionForHR(Vol().SetMute(value, System.Guid.Empty)); }
  }
}
'@

Usage sample:

PS C:\> [Audio]::Volume         # Check current volume (now about 10%)
0,09999999
PS C:\> [Audio]::Mute           # See if speaker is muted
False
PS C:\> [Audio]::Mute = $true   # Mute speaker
PS C:\> [Audio]::Volume = 0.75  # Set volume to 75%
PS C:\> [Audio]::Volume         # Check that the changes are applied
0,75
PS C:\> [Audio]::Mute
True
PS C:\>

There are more comprehensive .NET wrappers out there for the Core Audio API if you need one but I'm not aware of a set of PowerShell friendly cmdlets.

P.S. Diogo answer seems clever but it doesn't work for me.

Use the following commands on a ps1 powershell script:

$obj = new-object -com wscript.shell 
$obj.SendKeys([char]173)