FFMediaToolkit

public void ConvertMp4ToMp3(string fileFrom="some.mp4", string fileTo="some.mp3") 
{ 
   using (var mediaFile = MediaFile.Open(fileFrom))
   {
       var audioEncoderSettings =
           new AudioEncoderSettings(mediaFile.Audio.Info.SampleRate, mediaFile.Audio.Info.NumChannels, AudioCodec.MP3);
       using (var file = MediaBuilder.CreateContainer(fileTo)
                  .WithAudio(audioEncoderSettings).Create())
       {
           while (mediaFile.Audio.TryGetNextFrame(out var audioFrame))
           {
               file.Audio.AddFrame(audioFrame);
           }
       }
   }
}