It'd perhaps be more interesting to start off with a minimal implementation of Core Audio, considering there's a lot more existing code written around Core Audio?
I still need to dig into AVFoundation on OS X; on iOS I've only played with AVAudioPlayer and (although extremely easy to use) it's less than useful for anything except playing either a single long stream or an occasional sound effect. What they introduced in iOS 4/Lion seems an improvement, but Core Audio might still be a good basic API for developing AVAudioPlayer.
Yeah, I wrote that back when I didn't really know what was up or down. I went with the source/sink architecture because it seemed like the right thing to do. The code in NSSound is a bit of a mess, though. I would like to eventually revisit it, but that doesn't seem likely in the foreseeable future.
That said, there are really only 2 sinks that I know of: libao and OSS. Libao will use whatever the OS provides, be it pulseaudio, alsa, or something else. I wrote it as a catch all and as an example for anyone wanting to work with it. The OSS sink was written by David for FreeBSD... I never tested it.
NSSound is not a multimedia framework, though. Apple recently came out with AVFoundation, which looks really interesting. Would be nice if we could get an implementation of that on GNUstep.