I would like to suggest adding volume control for right- and left- speakers seperatly, allowing for "motion sound", like a train passing by etc.
Maybe you could try something with the PlayAmbientSound (int channel, int sound, int volume, int x, int y) function...
But you also can create a stereo sound with some programs like WavLab or something else, and import it in AGS.
Ok, I'll look into how easy this would be to implement.