Something I saw : when you play a mp3 with the ipod widget in background, you can play sound SFX on top of that when using an audio sprite. So, for the when, if you want background music, you need to provide audio on a different link, let the player add it to his library, start it, and play the game...
There's also a plugin for Phonegap that allows to use native calls to load and play audio (transparently). I need to find back the URL for that plugin...