How can I get "protected" music files to work with iMovie?

So I was in the process of making a movie for my parents and I had already added this perfect, classic song before updating to Mavericks. Now having switched to Mavericks (iMovie 10.0.1), it no longer plays and I see this notification saying 2425 files are protected and unavailable. I don't understand why I can't use 2425 songs I've purchased for use in my personal home video movies.

Is the only option to revert from Mavericks to what I had before? Is there any way I will ever be able to use these "protected files" in iMovie again?