Madplay allows you to play a MP3 in the background.

Release notes:

Place the scripts somewhere on your SD-Card and add them in Gmenu2x.

You need to set “DON’T Leave” Option on the Icon of the “madplay_play.sh” Script! But only there! Otherwise it won’t play.

http://sience.schattenkind.net/dingoo/madplay_background.zip

http://boards.dingoonity.org/dingux-releases/madplay-play-mp3-in-background