When your app is in the background it no longer has access to the CPU so your audio will stop playing.
The only way around this is with background agents running the audio component. The issue here is that the certification process will be hard on you if you are just playing looping audio. Playing audio in the background is intended for audio player apps (like the inbuilt "Music" app).
If I were a user of your app I would likely be unhappy that it clogs up the audio system when it isn't in the foreground (if, for example, I went to answer a Lync call). If the only way to stop your app playing audio is to go and turn it off manually or exit the app then my opinion is that the user experience isn't great.
Of course, you may have a different opinion, or your app might be doing something I haven't considered.