I found a class that seems to have all the capabilities I need to create a 3D sound library for Android. It is called
AudioTrack. The "write" method can be used to play PCM audio data, and the "setStereoVolume" method can be used to take care of gain and pan. It'll just take a little trigonometry - I've already done something similar with with JavaSound, so that part shouldn't bee too difficult. I will need to run some tests to see how well Android is able to mix multiple playing AudioTrack instances. If it can handle a reasonable number, then I should be good to go.