Tales of Phantasia on SNES has one of the most technically impressive soundtrack on the system IMO (and one of the best in general). And also happens to be one of the most offensively botched jobs on the GBA, a complete disgrace.
Exactly my feelings ! Even the PSX port is already a letdown from the SNES version sound-wise. Maybe that version would need to be sound-restored instead ?
If at all, I'd recommend an audio restoration of Mother 1+2, since that one is getting a lot of love lately with the VWF hack, and I'm planning a colour restoration of it.
Actually this is planned, by the group which is doing the translation. They will handle the work, not me though, since I know squat about the game
But only if you really wanted to do it for your own curiosity, and if you didn't have another GBA game restoration in mind.
Actually there's not much other sound restorations I could do. Maybe Mega Man & Bass, but to be honest I like the GBA music and I can't say they butchered it. On the other hand, they used the GB sound cleverly, and it fits the soundtrack well. Should have been an example for other SNES->GBA conversions... I was also thinking about porting the music from Mega Man Battle Network 5: Double Team (DS) to the regular Mega Man Battle Network 5 GBA, but I don't think this would be any useful other than a feasibility tech-demo (and I am already convincted this is feasible).
DKC does't use sappy so in order to do a restoration major work would be involved in craking the sound engine it uses, and make tools that convert the SNES songs for it. Not impossible, but that's another scale of work than just applying current findings to games using the sappy engine.
Slow battles as well. I don't know how much of this is the fault of intentional programming versus the system struggling to run the game due to poor optimization.
It's hard to tell but if the sound mixing is in cause, it's easy to shut sound mixing off entierely and see if the game stops slowing down or not. We can also observe in NO$GBA emulator the CPU usage gauge, and when the game's lagging we see the gauge flickering between 100% usage and a small usage (because each "frame" takes 2 frames to actally complete). If CPU usage is below 100% we know it's intentional prgramming.
EDIT: Just checked it, the CPU usage is always 100% during battle... very strange.