SpritesMods had published a guide to using a GameBoy Advance as a MIDI synthesizer:

So, does it work? Quite well actually. At first I was planning on just converting the 4 GameBoy Classic sound channels to MIDI, but it seems I’ve ran away with the concept a bit…

The end result is quite nice, though, there are lots of sounds to be gotten from the little GBA and it actually has become a fun-to-use little and cheap synthesizer.

See the SpritesMod site for details. The firmware for both the AVR- and the GBA-code is open-source and can be downloaded here.