Quoting @Mark from another thread:
If you double click the "eye" icon on the movie player, and enable 'audio device', 'snd out' and 'audio trks', you'll see these options.
Right now, these channel related inputs currently only function on MacOS if you set the mode to 'interactive' -- which means you are using QuickTime to play the movies.
If you set the number of 'audio trks' to three, the Movie Player looks like this.
The audio device input allows you to send the sound to a specific output device. The 'trk' inputs specify the volume, panning, and routing for each track.