I removed my gba music tool from download page, because it has some conversion bugs and it will never get updated.
Today, there are some nice tools which can replace old gba2midi tool. I recommend GBAMusRiper, which can convert GBA sound into MID and SF2. Moreover, you can download the source code.
- GBAMusRiper - VGMusic (Java)
- VGMTrans - Nintendo DS Hacking Wiki (C++)
- Download Sappy 2006 mod 17.1 (Visual Basic 6.0) (Updated version of Sappy, which probably has some nice features in ROM hacking)
Their source codes are good reference for understanding MusicPlayer2000. Enjoy your reverse engineering life.