Thanks Ron, Matthew, Larry, Edisaurus and Dan (the Man) for questions and things to think about. Let me see if I can answer some of the questions:
I didn't have a specific inventory of what I had in the boxes. The musicals will actually be easy to figure out. They were in alphabetical order in the boxes and I can make a list of what I still have...then just try and remember what's missing. I hope that that'll work. The DVDs, however, will be harder. I know some large sets that I owned (the complete "Space 1999," four seasons of "Soap," things like that), but I don't remember everything that I had in there. When I was going through a couple of boxes that I still have, there were some titles that I didn't remember buying at all.

My garage was locked but I don't know when/if I ever left it unlocked. I wouldn't think so because I have to make sure that it catches when I close it. It doesn't always stay closed the first time I shut the door, so I always have to check it and make sure.
Otherwise, I don't know how they got into my garage. My building was re-roofed a couple of months ago and they did take off the roofing material over the garage, but if someone from that crew did take my CDs and DVDs, others on the crew would've known about it and then they all would have to be in on it. That just doesn't seem plausible. When I told the property manager, she said that she trusted the workers very much. That doesn't mean that they couldn't have done it, but it just doesn't seem very likely.
I called one local used CD store, and the guy I talked to said that he did most of the buying for his store. He didn't remember anyone coming in with any cast album recordings anytime recently. Of course, that was only one store, so you never know.