before getting into demagnetising, isn't the cheapest way to test this actually to do the opposite and try to magnetise something to see if there's even a problem to be fixed  - ie take a new cd (which we assume isn't magnetised yet...), play it then magnetise the buggery out of it  - maybe with a magnet..! - play it again and see if it sounds any worse/different?    I'll have a root round in the kids' toybox to see if they still have a magnet and if so give it a whirl..