Public CVS is more up-to-date than our 0.6.0 release , so if you're going to check it out, get it from CVS.
It mixes multiple streams, can convert sampling rates, and doesn't need a half-second long buffer. We haven't got it to slice and dice yet, though.
The API that is in there is VERY low level. We've been working hard on a high-level "soundserver-style" API that'll allow esd and OSS apps to be ported easily. It'll be out when-it's-done, which will be soon.