If you have a 4 speaker system hooked up or some other type of configuration, the sound inputs can be reorganized via software. So check your sound, in control panel (which you probably already did)or installed sound widget. All I can think of right now...
Also, what kind of sound hardware....board...cpu...software or game you are trying to use...etc.
If you are using this for a game, make sure the game config for sound is working i.e. half-life mic executable mic setup.