GBA is a lot harder! On GB/GBC the music is just code, so if you can find it, you can execute it. This also means you just need to store a map of where it is (e.g. pointers) and not any intellectual property. On GBA and SNES, devs had access to more modern codecs, so they might just store music as a wav or mp3-like file (or a proprietary one), which makes the job a lot tougher. The hardware and software I built is setup so that I can rapidly move on to NES next. For me, NES is the sweet spot of quality of music and ease of extraction.