There is an option in the Wii settings for surround output. Then again it is only running off a stereo output cable. Mine was in surround when i played CoD 3 though..
Nintendo Wii supports Dolby Prologic II. This (with a proper reciever) will decode the Red/White RCA plugs into 5.1 surround sound. Granted you will need a Reciever that decodes ProLogic II, and the speakers to boot.