currently FPSC doesn't have 3d sound (to the best of my knowledge, and I hope it's added in a future update), but what if you have 5 different versions of the external sound (see attached .jpeg diagram, each color of arrow represents a sound file). You could use a script to locate the origin of the sound and use the locate-angle type of commands (can't remember the exact ones right now), as well as their distance ---> sound-strength. Sure, when your turning it would either keep the same sound playing, or be too obvious of a jump to a different position, but there's a few ways to counteract that that i think could be done:
1) make a crap-load more variations of the sound (for a smoother "fade")
2) more coding, maybe do an actual fade between variations of the sound based on the player camera position
That Big Idea guy that can't find the time...