Exquisite work as usual Bregalad! You're amazing for doing this and i'm in 16bit heaven right now!
Your GBA music ripper program does indeed extract the music midi's from Mother 1+2 with soundfont so I'm assuming that means it does indeed use Sappy and could be restored if someone wished. That is how you can tell whether a game does use that engine right? If your program rips music from a GBA rom it uses Sappy, if it doesn't it's not using it. I tested several popular GBA games (mostly ports) with your music ripper just to get an idea of how many use the Sappy engine. While it's a shame so many SNES-to-GBA ports ended up sounding terrible in the porting process, I guess it's almost a blessing so many of them DID share the same sound engine. With your programs and documentation, seems like its made it perfectly possible for people to make their own sound restorations. Haven't seen any other attempts at GBA sound hacks besides yours yet sadly, but I hope we see more of these hacks in the future.
Actually ran into several popular GBA games that apparently do NOT use the Sappy engine though in my various tests. At least ones that your music ripper can't seem to find the engine in. Your program can't find Sappy or extract any midi's from Mario Advance 2, 3, or 4 (Mario World, Yoshi's Island, and Mario 3). The only Mario Advance game that it was able to extract music from was the original Mario Advance (Mario Bros 2). Shame because they could all use better quality music, especially Yoshi's Island. Though Mario 3's sound isn't really that bad compared to many other SNES ports i've seen.
Rayman Advance doesn't seem to use Sappy either. I realize Rayman was originally a CD-based game that used more orchestral music but the sound quality is so bad on the GBA that it would at least have been interesting to hear its music in somewhat better quality sound (with SNES-like instrumentation). Oh well.
None of the Donkey Kong Country games appear to use the engine either. The music in all three games is a real tragedy but to be honest I don't think anyone would bother with them anyways. There's a lot more wrong with the GBA versions than just the sound quality. Like how almost all the sprites and backgrounds are reduced to only 75% of their original size, the colors being all washed out, and many removed visual effects like day-night transitions and other lighting effects. Can't believe Rareware themselves were responsible for these ports (i'd only loosely call them ports, more like bitter retoolings)...
Tales of Phantasia seems to use Sappy though. Though similar to the DKC ports, there's a lot more wrong with this version than just the sound quality so that most hackers probably wouldn't think it worth their time. Laggy battle engine, removed or dumbed down visual effects such as reflections/shadows/lighting, etc. Pretty bleh all around. Not to mention the horrid English translation.
Both Breath of Fire ports use Sappy though, so sound restorations might be possible for those. As of this time though there's a really great retranslation patch for Breath of Fire 2 on SNES, so i'm not sure whether anyone would even want to bother with it. Besides the sound quality though both are pretty darn good ports of the SNES originals (there are brightness patches for both too, though I lost and cannot find the one for BOF1). The Magical Quest Mickey Mouse trilogy also appears to use Sappy. They're pretty good ports outside of their sound quality (2 and 3 even include color brightness settings which make them appear close to if not as good as the SNES originals on backlit devices, something I wish all GBA ports had gotten instead of completely messing up the colors on an internal level). The sound quality even improved drastically with each game. Magical Quest 1 though has some really low quality music.
By the way, am I also to assume it would be possible to clean up the sound quality in original non-ported GBA games? The sound quality problem in GBA games is definitely not exclusive to SNES ports, a lot of good original GBA games are lacking in the music quality department. For example, the Megaman Battle Network series and Castlevania Harmony of Dissonance all seem to use Sappy according to your ripping program. They're also all lacking in the music quality department IMHO. Would it be possible for someone (using your documentation and tools) to improve the music quality in them by swapping out superior sounding instruments, making them sound closer to something on the SNES even without actually being ports of SNES games?
Otherwise great tunes like these could sound a lot better-http://www.youtube.com/watch?v=cLrb__UnlcAhttp://www.youtube.com/watch?v=c5tSgqPLbpUhttp://www.youtube.com/watch?v=8ENmrNA6ifc
What if they sounded closer in quality to these? (granted I know there are limits, but I feel improvements could be made)-http://www.youtube.com/watch?v=y-A1NWI5Y1Uhttp://www.youtube.com/watch?v=ouwGpIW9Twkhttp://www.youtube.com/watch?v=2zsV113dfnI