Decompiled as in turned back into a high level language? Maybe in theory (though a lot of games still used inline assembly) but nothing in practice. X86 family machines being the main thing being looked at there, though I suppose we did recently see Mario 64 done.
Decompiled as in like games on CD/DVD or newer cart systems where you fire it through a program and it breaks it down into constituent files? No. Some GBA homebrew has such functionality but commercial games did not. The sappy audio format mentioned at the top is enough pointers and detectable format that you might get a program resembling it, and there are a few games that people did put the effort into (surprisingly though Zelda Minish Cap is not one of those -- Zelda games are usually some of the more noted so no idea what goes here, guess those kinstones really spoiled it for people). Granted this would mostly be useful for 1 through 3 on that list of things I suggested.
Jorpho mentioned disassembly, or a fully commented disassembly as some would term it. For the GBA I think we are still looking at just a couple of pokemon titles that got this kind of treatment ( https://github.com/pret/pokeruby
) and even then it is nothing like the older titles there in what you have to play with.
As Jorpho also mentioned your job would be to follow the GBA's efforts as it produces the sound. You could try going backwards from the sound itself -- sound itself is its own hardware/memory section after a fashion ( http://belogic.com/gba/
) so it will eventually tickle that. Find what does it and work backwards from there.
Myself for this I would probably go forwards. Find the actions that cause the voice effects (be it a button press, triggering a cutscene, reaching a location, idling if there is an idle animation...) and go from there -- in among it all will be hit detection, animations, damage calculations and on and on and on but in there will be the sound call. You don't have to understand everything it does during this, indeed you will probably wait for something to play with the sound memory area and look more closely at that, and only need to worry about the sound. How you will stop it once you get there I don't know as it will probably vary between games but the main one would be NOPing the sound call and seeing what happens, of looking what happens at the end of it and jumping directly to that location.
As for accomplishing through hex then technically speaking as you can edit any part of the ROM you can do any hack with it, but from a practical perspective you don't do that.