Personally I would go for emulation. Plenty there do fun things with sample rates (being an emulator you can do what you like after all) to go beyond the hardware. There were some going there during the GBA lifetime with ports of VBA but I don't know what the kids and golden ear types from HCS64 https://hcs64.com/mboard/forum.php
and all the audio ripping scenes are using in current space year.
That said the link port is pretty slow so it would be some kind of note on, note off midi style setup (which is not a world away from 95% of GBA audio -- the GBA does have wave options available to it, nowhere near as much as the DS has but you can play back wave audio* and some things do, but sappy and many of the other formats like krawall https://knzl.at/krawall/ https://github.com/MCJack123/UnkrawerterGBA
*I could cover GBA audio hardware, and will elsewhere, but easier to start with http://www.gameboy-advance.net/emulated/musicplayer_advance_gba_mp3.htm
. Neither are the cute thing of abusing samples stitched sequentially either.
Personally I would either attack it at sappy level if that is a thing, or sit on the audio hardware ( htttp://problemkaputt.de/gbatek.htm#gbasoundcontroller http://belogic.com/gba/
) and duplicate whatever it does as link port packets** before having something like a teensy or arduino redo it as a more conventional format. There have been some people that attempted various levels of midi controller for this family but that was usually more on the DS side of things (DS midi wifi and DSserial stuff being among the more notable there). However scope for things exists http://www.brolinembedded.se/projects/keyboard/ https://web.archive.org/web/20120628204500/http://www.reinerziegler.de/GBA/gba.htm
and all the fun with various multiboot stuff (whether you want to do something like vbalinkreal but for audio vs just having serial data handled like any other serial data I don't know).
**this is how a lot of assembly additions work. Find something that is at least triggered by the same thing you care about and add a little detour along the way.
You will also face the sample quality issue as even with the best will in the world you are still going to suffer those (32 megabytes, though for most practical purposes less than 16 means you dropped sample quality as a matter of course -- even on the DS I have otherwise been talking up throughout this then much of it is still in the 11KHz range) so unless you are going to go in for something like the donkey kong music restorations; https://www.youtube.com/watch?v=WW--iJYcZNE
people discovered the sample library from the original composer program and redid things for that, as well as fun like https://www.youtube.com/watch?v=bsCN0Yx2Vbs
which is more for graphics but better illustration of the kind of mindset/workflow likely needed there. This also says nothing of the composers maybe actually realising it sounds like it is at the bottom of a well and playing to that; http://bogost.com/games/a_television_simulator/
for the video equivalent.