Steve's method is the best and quickest if it works. Also try putting tape over any of the holes that let the sound out. If you want to open the thing up, you might find the sound-making device and be able to e.g. put tape or some more mass on it to reduce the sound. Or wrap some foam around it. I'd suggest that electrical modifications be last on the list, as it might be easy to break something if you're a noob.