So yeah, playing mp3s has a hit on performance as result; having lengthy uncompressed music is bad for filesize and not ideal for ram etc either.
So yeah, due to these and also out of convenience for the user (i myself often listen to podcasts during playing other games) one of my most wanted features for unity iphone is a feature that allows one to check if ipod is playing and if so be able to react on that (continue playing ipod and only play sfx, no ingame music, turn ipod off and play own music etc).
One thing i was worried about was whether that would cost a lot of performance hit for the game if the ipod app is playing during the game.
Well, today i noticed something:
Was plying ipod and started the unity app, then of course the ipod faded out. But then i pressed the clicker of the earphones and the ipod started playing again (turning off any of the sound side of my game in return). The nice side is that the game runs pretty much the same besides that. So yeah, that´s pretty nice and made me happy thinking if its supported on unity side in the future to not stop it in first place it can run it good performance, just thought maybe others would like to know, too 
I think you can also do this by double-tapping the home button while the game is running. This should bring up the music play controls as well as a volume control.
sweet, i had no idea
Regarding just keeping the iPod playing in first place from as far as i understood it it gets turned off because unity iphone doesn´t handle sfx and music playing seperately in different ways but instead for both uses the one way which makes it not possible to have the iPod playing and then the only way to have the iPod playing would maybe be if one does a postProcess build thing where one comments out the initializing of the unity sound handling,no?