Request to add additional commentary tracks to play synchronously with movies and videos either by native functionality or via plugin. I have recently dicovered RiffTrax and RiffPlayer that plays DVD synchronously with an mystery Science theater 3000 style audio commentary. It plays a 2 channel MP3 along side the regular audio track. This works well, however, it doesn't work with blu-ray or video files, So you have to resort to manually synchronizing two media players to get the same result. It would be allow the plex player to buffer from time to time and not have the user have to manually resync.
I would imagine that you would have to down mix the audio from the movie to 2 channel and mix in the MP3(or other format audio) to play nicely with mobile phones. For PC/Mac you could just stream two synchronized LPCM streams and let the browser or native audio mixer do the rest. For the handling of audio levels, that would me more tricky.
For audio levels on mobile you could have preset volume leveling. for example, commentary track +/- 15%,10%,5% or 0%.
For PC you could add a slider for equalization.
Additionally, You could collaborate with the guys at Rifftrax.com to figure out the details as well. I am sure that they wouldn't mind having a channel/plugin that would allow them to get more coverage for their products.