The sound format for GBA (as far as I know) is GSF. You can get some info and tools here:

http://gsf.caitsith2.net/
http://gsf.caitsith2.net/ripping.html

You'll most likely have to use a debugger (no$gba, I'm not sure if vba-h supports sound breakpoints) to find the locations and make sure the sound engines are the same. I haven't done any sound hacking before, so keep that in mind when reading my answer. You could also probably read some documents about sound hacking for the SNES as the concepts are probably fairly similar. Good luck!

Thanks! I also got it working on sappy.

Two quick points before I disappear back into the mist:

1. GSF (like all the *SF formats) isn't strictly an audio data format - GSF files actually contain GBA ROMs that have been hacked so that all they do is play audio - the rest of the data/code from the original ROM that isn't used gets zeroed resulting in a smaller file when compressed. HighlyAdvanced (a GSF player) is pretty much just a stripped down version of VBA.

2. The version of VBA-SDL-H on this site doesn't do I/O breakpoints but the one here does. When I get some time off work later this month I'll get things updated.