Player-originating sounds still only play in mono. I have prepared a test wad to demonstrate this. Simply load the wad and start a new game, and then jump. The resulting sound should test the stereo sound channels - in the left speaker, it should say "Left channel", and after that, the right speaker should say "Right channel". As of the r858 build linked above, the sound currently plays in mono, meaning both speakers will be hearing both messages.
GZDoom 1.1.0, which uses the old FMod.dll, does not have this problem. Each sound only plays in its designated speaker.