I'm reluctant to use Gumby's SciAudio as I want to have the game files as small as possible.
Yeah, I wouldn't use that then. If you are worried about your game size, it'll allow you to use mp3s instead, but the way it was implemented chews up a lot of heap space - just ask Doan, he pushed it as far as it could reasonably go with Betrayed Alliance.
Did SciAudio ever get moved over to use the Sound class? Or does it still require windows to run?
As far as I know it requires Windows to run. I think at some point ScummVM added support for it, but I could be misremembering. It'll never be 'native' in SCI games, it'll always be something that the operating system takes care of, external to the interpreter. I did toy with the idea of tying it into the Sound class, but never did so - either way it doesn't change any of the implementation, adding it to the Sound class would have just made using it more consistent with the scripting language.