Various audio libraries extracted from my Praxis software, and now in use in a number of other commercial and open-source projects. In particular, the AudioServer API provides a backend-neutral callback-based
audio API - think of a PortAudio equivalent for Java that separates your code from the underlying audio system.
Current backends available are JACK and JavaSound (the JavaSound backend includes a variety of hacks to improve performance, while letting you work with the simpler callback based API). A PortAudio binding is in the planning stages, and various other things are under discussion.
NB. The AudioServer API is not a mixer, though you could easily build one on top of it.