are you by chance plugging it into an old receiver that you have set to PHONO?
it applies approximately 70db of gain to the signal and that'd do what you're describing, other than that, if your TV is amplifying the signal that'll mess it up too, but I've never seen a TV with anything other than LINE level inputs.
I'm not sure if it was somebody misunderstanding the WiiMote volume controls but I think it might be possible to set the volume of the wii itself, I dunno, seems unlikely but if so that'd handle your problem as well.
Is the signal cutting in and out and kind of garbagey, or does it sound like it's too loud and distorting for that reason?
Before you try anything too drastic, try cleaning the contacts on your TV/Receiver as well. Just swab around the outside and the inside with a cotton swab dipped in rubbing alcohol. Then if it still doesn't work, tip the bottle and forget your problems :P