code: public T setAudioChannels(int channels) {
      checkArgument(channels > 0, "channels must be positive");
      this.audio_enabled = true;
      this.audio_channels = channels;
      return getThis();
    }
docs: Sets the number of audio channels
  
  @param channels Number of channels
  @return this
  @see net.bramp.ffmpeg.FFmpeg#AUDIO_MONO
  @see net.bramp.ffmpeg.FFmpeg#AUDIO_STEREO