A musty smell is really a mold and you need to kill it off. If you have an automotive body/detailing shop around you, some of them deal with cars that have gotten moisture issues. What they do is they ozone them. Maybe try just phoning around, see who does it and then drop around and chat with the boss. Explain your hobby and what your problem is....they might be amenable to giving you a call when they are going to do a car and you just put the album covers in the are and let the ozone do the work.