Use Winamp or your favorite media player to convert your MP3 music into WAV. In winamp, for example
use the DiskWriter plugin. Then use Audacity or Cool Edit to split the stereo WAV(s) that you now
have into 2 each of mono wavs with left and right channels...

THIS NEXT STEP IS IMPORTANT:
To get it so the proper channels go in the correct places and you have the correct track order, name
each song as follows.

Original WAV song:	Split WAVs:
CD Track One.wav		01LCD Track One.wav
				01RCD Track One.wav
CD Track Two.wav		02LCD Track Two.wav
				02RCD Track Two.wav
CD Track Three.wav	03LCD Track Three.wav
				03RCD Track Three.wav

Save the split wavs in the "wavs" folder, then double-click Go.bat

You will now end up with a dpcm.GBA file which has stereo music on it. Run this on GpSP or a real GBA.
(It sounds pretty broken-up in VisualBoy Advance CE.)