[Zubersoft]

The existing audio libraries I have at my disposal do not have support for stem mp4 files. I would need to have access to open source libraries on Android, Windows and iOS before I can consider supporting this. I did not find any Android libraries while searching.  Android applications can only communicate with other applications through things called Intents. Unless Audacity has specific intents it can use to allow other applications to play audio through it in the background, I doubt that would work. Most Android applications are not really designed to be used that way unless they are a background service. 

Mike