string inputVideoPath = Path.Combine("C:", "Temp", "input.mkv");
string outputPathMp4 = Path.Combine("C:", "Temp", "result.mp4");

IMediaInfo info = await FFmpeg.GetMediaInfo(inputVideoPath);

IStream audioStream = info.AudioStreams.FirstOrDefault()
    ?.SetChannels(2);

return FFmpeg.Conversions.New()
    .AddStream(videoStream, audioStream)
    .SetOutput(outputPathMp4);