GBATEK, an extensive documentation of the GBA hardware: https://problemkaputt.de/gbatek.htm
Instructions on how to compile GBA programs on Linux using the GCC compiler: https://gist.github.com/JShorthouse/bfe49cdfad126e9163d9cb30fd3bf3c2
Tonc, a guide for GBA programmers: https://www.coranac.com/tonc/text/toc.htm
A guide explaining the basics of Audio Programming for the GBA, focusing on Direct Sound: https://www.gamedev.net/tutorials/programming/general-and-gameplay-programming/audio-programming-on-the-gameboy-advance-part-1-r1823/