It should be worth noting that the sound issues aren't a bug, per se, so much as a side effect of the game running slowly. mGBA doesn't do any sort of audio stretching, so what you're hearing is the sound coming out as fast as it can, and gaps when it can't keep up, which leads to the awful mess you hear. Trust me, I know it's horrible, but there isn't a real fix apart from speeding up the emulator. When games run full speed, the audio is clean, but very few games run at precisely full speed, apart from GB games. (The frame limiting bug was fixed a few days ago. If you're still getting it, download a new dev build.)
I've been working on making mGBA faster recently. Through two minor fixes recently, it's gotten about 3% faster, but that's nowhere near the performance gain that will be needed to bring mGBA to be full speed on the Vita, and I've run out of places I can get big gains. I've also started work on a dynamic recompiler; however, I seem to be utter trash at optimizing a dynarec so it's currently about 5-20% slower, depending on which game you use. That said, I'll keep developing it and hopefully it'll be up to snuff eventually. Also the dynarec only works on Linux at the moment.
The real reason it's so great on the Wii is that the Wii has juuuust enough horsepower to run basically all games at full speed. But it's also a 729MHz PowerPC core, compared to a 444MHz ARMv7 core. I don't know how the IPC stacks up against these chips, but needless to say, the Wii is a much faster processor overall.
