From here: Windows phone 8 on emulator - why can't I play audio files? see the answer number 3 (Author:Frederik Winstrup Johansen). He also added a good sample about playing AudioTracks
The BackgroundAudioPlayer can play files only from isolated storage or from a remote URI, that is why you can here anything!
If you have your file as resources in your app, you must first copy them to the isolated store, and then make a reference to the file in the isolated store to your BackgroundAudioPlayer.