Short answer : No you can't have music, but I'm hoping to find time to add it in soon.
Long answer : Yes, you can play mp3s in the background, but there's no point as it slows the game down a lot and knocks out the sound effects. I might add the ogg decoder that runs on the second cpu at some point (but that will probably slow things down a bit still), but before that I plan to get it to play wav files like I did in the gp32 port. You'll need to knock the quality of the wavs down a bit to keep the file size reasonable, but on the gp32 it still sounded ok and had very little hit on the framerate.