C# Download the sound of a youtube video

Install the NuGet packages: MediaToolkit and VideoLibrary, it will allow you to do the conversion by file extension.

var source = @"<your destination folder>";
var youtube = YouTube.Default;
var vid = youtube.GetVideo("<video url>");
File.WriteAllBytes(source + vid.FullName, vid.GetBytes());

var inputFile = new MediaFile { Filename = source + vid.FullName };
var outputFile = new MediaFile { Filename = $"{source + vid.FullName}.mp3" };

using (var engine = new Engine())
{
    engine.GetMetadata(inputFile);

    engine.Convert(inputFile, outputFile);
}

The above code works awesome you don't need to download the video first I created this procedure so when rookies like myself see this makes it easier to use. See note above about nuget packages need.

example url: https://www.youtube.com/watch?v=lzm5llVmR2E example path just needs a path to save file to. just add the name of the mp3 file to save

Hope this helps someone I have tested this code;

private void SaveMP3(string SaveToFolder, string VideoURL, string MP3Name)
{
    var source = @SaveToFolder;
    var youtube = YouTube.Default;
    var vid = youtube.GetVideo(VideoURL);
    File.WriteAllBytes(source + vid.FullName, vid.GetBytes());

    var inputFile = new MediaFile { Filename = source + vid.FullName };
    var outputFile = new MediaFile { Filename = $"{MP3Name}.mp3" };

    using (var engine = new Engine())
    {
        engine.GetMetadata(inputFile);

        engine.Convert(inputFile, outputFile);
    }
}

Ok found a better way the above code didn't normalize the audio posting it for others. First Add Nuget package: https://www.nuget.org/packages/NReco.VideoConverter/ To Convert MP4 to MP3

 // Client
        var client = new YoutubeClient();
        var videoId = NormalizeVideoId(txtFileURL.Text);
        var video = await client.GetVideoAsync(videoId);
        var streamInfoSet = await client.GetVideoMediaStreamInfosAsync(videoId);
        // Get the best muxed stream
        var streamInfo = streamInfoSet.Muxed.WithHighestVideoQuality();
        // Compose file name, based on metadata
        var fileExtension = streamInfo.Container.GetFileExtension();
        var fileName = $"{video.Title}.{fileExtension}";
        // Replace illegal characters in file name
        fileName = RemoveIllegalFileNameChars(fileName);
        tmrVideo.Enabled = true;
        // Download video
        txtMessages.Text = "Downloading Video please wait ... ";

        //using (var progress = new ProgressBar())
        await client.DownloadMediaStreamAsync(streamInfo, fileName);

        // Add Nuget package: https://www.nuget.org/packages/NReco.VideoConverter/ To Convert MP4 to MP3
        if (ckbAudioOnly.Checked)
        {
            var Convert = new NReco.VideoConverter.FFMpegConverter();
            String SaveMP3File = MP3FolderPath + fileName.Replace(".mp4", ".mp3");
            Convert.ConvertMedia(fileName, SaveMP3File, "mp3");
            //Delete the MP4 file after conversion
            File.Delete(fileName);
            LoadMP3Files();
            txtMessages.Text = "File Converted to MP3";
            tmrVideo.Enabled = false;
            txtMessages.BackColor = Color.White;
            if (ckbAutoPlay.Checked) { PlayFile(SaveMP3File); }
            return;
        }