Links
General Gameboy Advance development info:
Good intro into how the Gameboy Advance works
Gameboy Advance Developers website (LOTS of stuff there)
The Nintendo Reverse Engineering Project
GBA Dev.org
GBA Junkie
GBA sound tutorial
Better sound tutorial
GBATEK - good technical GBA info
General Mp3 playing info:
LAME (open source mp3 encoder)
mpg123 (mpglib is open source mp3 decoder library)
Pinouts:
GBA cartridge/link cable pinouts
PDF file:Compact Flash Spec 1.4
GBA internals pinout
GBA cartridge pinout
FAT filesystem info:
Detailed layout with C structures
Hardware that may be of interest:
USB.org developers site
Lots of info on Flash linker sets
Building an EEPROM reader/writer (may be helpful)
Smartmedia vs. CompactFlash vs. others
Competition/colleagues:
Info on another homebrew mp3 project
SongPro (ad-based mp3 player, eeew)
Kemco mp3 player (can't find official site)