It was stuck in the previous forum incarnation but when the sections got merged it did not make the cut.
Anyway decompiler in the sense you probably mean it (like ndstool or the various CD/DVD based games but for the GBA) does not exist for commercial GBA games (homebrew might use the system or a similar one). GBA game data is mushed into one big blob.
For audio then most games (though not as many as the DS and its audio format) will use the sappy format. This has a few tools that can fish it out from the ROM.
Several people have looked at Donkey Kong games on the GBA, usually with an eye to making them closer to the SNES originals (as well as palettes there is also some trouble with reduced resolution things). You might find something more in those discussions, romhacking.net forums having several.