Shin'en GAX Sound Engine (GBA) Specification
The research is based on GAX Engine V3. Information about V2 can be found at the end of the document.
FYI: Ray1Map is a tool that can analyze GAX V3 data. You may find more information there.
FYI: You can find my IDA FLIRT signature for GAX V3 here. https://github.com/loveemu/ida-sig
FYI: Gaxtapper: Diagnostic tool / Automated GSF ripper for GAX Sound Engine.