For musty smells - clean it out as best you can and then throw some odor eater shoe inserts in there - the kind with charcoal. My DH does this when he gets an old radio that belonged to a smoker, it helps a lot. There's usually some smell left over but mild enough he can stand to work on it and the rest seems to dissipate with time and open air.
I've also heard of using wadded up newspapers to trap bad smells but haven't tried that myself.