Youtube will pick one track, probably track 1, for playback. Since you didn't assign any source to track 1, Youtube is silent.
Media players will pick one track at a time only. You need to switch between tracks within the app. No media player will mix existing tracks and play back them all at the same time.
Multiple audio track recording is for postprocessing, where you manually mix all recorded tracks with a postprocessing tool and finally replace them with the one mixed track.