I posted about a release by Pixelh8 earlier for the DS but he also released another piece of software for the GBA/DS:

Pixelh8 Music Tech Pro Performer Synthesizer is is software that runs on an unmodified Game Boy Advance or Nintendo DS, enabling you to be able to control the on board sound, turning the GBA/DS into a real time instrument. Based on the original Game Boy Music Tech V1.0, Pro Performer offers hundreds more options and possible sounds.

New Features
All of the features of Music Tech V1+ More!

Nineteen Pitch Sweep Options , Wave Duty Selectable 12.5%, 25%,50%,75% ,
Seven Volume Envelopes
All Major Keys , All Natural Minor Keys, All Harmonic Minor Keys, All Whole Tone Keys
All Pentatonic Keys, Primary & Secondary Key Select
Key Change in Player Mode, Octave Change In Player Mode
Maj 3rd, Min 3rd, 5th’s, Octaves & Unisons
Portamento Mode, Tremolo Mode, Arpeggio Mode (Up to Six Notes), Shifter Mode
Half Tremolo Mode, Vibrato Mode
Special Mode Speed Control, Unison Mode Phase Control
More Sound Effects!
& Hidden Programs, Presets & Modes!!!

Notable owners of the Pro Performer Imogen Heap (click to see video) & Damon Albarn.

The author states on his site that he will use legal proceedings against sites distributing his work but it seems some sites take a disregard to his request but im sure he will know who they are soon enough

But anyway download Here --> http://pixelh8.co.uk/software/