As far as I understand the mechanics of game sounds, they should always be monaural. Why, you might ask. Well, lets think real life things first. If you sit in the cockpit of a plane, you hear the engine roaring in front of you. One engine, not two. Or in case of a P38 two engines, one on the left and another one on the right. If another aeroplane passes you, you'll hear one aeroplane coming and going. As all of you surely know, mono stands for one and stereo for two. The idea of stereo is to give kind of a location for one single sound source like an engine. Now if you have a stereo sound of an engine, it most probably is locating the engine right in front of you as if you were in the cockpit. Or in case of another plane and engine you'd be head-on with the other plane. What if you turn your virtual head? Where would a stereophonic sound be? Either way off or still sounding like the sound source were right in front of you.
It's the game engine that calculates where each single sound source is located in the virtual 3D landscape, which way it's moving and even making note of how your virtual head with the two virtual ears moves. The result is a stereophonic, or even better, a multichannel sound telling you aurally what to expect and from where.
Regular stereo sound recordings are intended for giving an illusion of a 3D soundscape where the listener is sitting still in one position. The best results can be reached either with eyes closed or looking at a picture or movie taken at the recording spot like e.g. in a concert or at a theatre.
