What motherboard/soundcard do you have? Sometimes in the volume propieties theres a 'volume' for 3D Stereo or something like that, very common when using SoundMax drivers. Check for that 3D stereo and deactivate it or just mute it. If you don't see it, go to options -> Propieties. Check the list and activate sime 3D option and change it's settings.
I'm talking about this volume window: