FMOD assumes both channels originate from the same position when a stereo source is placed in 3D space. Ideally we would be able to set a stereo spread width when placing 3D stereo sounds. This is a feature just added in UE4.11 (Stereo Spatialization)…
I have asked here if we can get this feature in FMOD (as well as the rest of the audio features just released in 4.11)…