AE and 3D Sound with Tracker?

The best way to generate 3D sound effects is to render the comp, add it to Premiere Pro, then edit the sequence in Adobe Audition. You have the option in Audition to accurately position sound in several different scenarios. There is even a workspace dedicated to that kind of stereo mixing.

If you just want to control balance and volume based on 3D layer position and camera position it's going to take some vector math and some interpolation methods. I have not tested the sample expression that ShiveringCactus posted but it looks close to volume. For balance left and right you would calculate the angle from the camera and the distance. I don't have time to try that right now, and I'm not good enough to do write that kind of code without having AE open but I'll bet Dan Ebberts could do it.

Sound is not processed linearly, so aside from the geometric layout the expression would have to account for volume based on a logarithmic scale plus you need to figure in doppler effects, indirect echoes and a number of other things. None of this exists in AE and you'd have to re-create it yourself more or less. Otherwise, most of such stuff is still mixed traditionally for movies and even games do not necessarily produce environmental sounds, but rather just use audio triggers to mix conventional tracks. Even going through the trouble of setting up such a complex system in AE may not get anywhere near producing production-ready, "realistic" sound. I'm pretty sure you'd be rather disappointed how dull and boring the result might turn out.

Mylenium

@Mrtn Ritter is right, but... you could use the length expression to control the volume of the sound source by the proximty to the camera.  Something like:

VolumeControl = 10;
vol = value[0]-length(thisComp.layer("Camera 1").transform.position, thisComp.layer("Null 1").transform.position)/VolumeControl;
[vol,vol]

This takes the value of the audio file, and minuses the distance between the camera and null object.  The volume control value divides this length to drop the numbers down which flattens the waveform.

You cannot place a sound source in space in AE, unlike Unity. But you can use a Null to drive balance and volume using expressions.

*Martin