Out of the box, only JavaSoundMixer is supported, which is pretty feature-free and also buggy.
There is JOALMixer growing in the incubator (https://j3d-incubator.dev.java.net/
) maintained by David Grace.
It looks promising so far, but also still have a lot of issues. I assume that there many issues on the Java3D side (so not in the mixer itself). I'm in contact with David to bring it to work for FlyingGuns, but theres still some way to go.
The current state of sound in Java3D is: don't use it!
If you have the time, it would be a good idea to contribute and support David (thats what I do). If you just need a sound solution, you'd better roll one yourself.