How can I unlock protected aac files?

If iTunes can’t recognize them when you add them back, they’re probably corrupted. Otherwise, you should be able to add them to iTunes an authorize your computer to play them. Just my two cents.

FYI unlocking protected AACs may violate copyright and Terms of Service with Apple and iTunes; it is best that you contact Apple’s iTunes Support, which I think you can do from inside iTunes; I have lost files in the past, and they were pretty accommodating in allowing me a ‘one time’ re-download of the content.

In your case, I see it’s not an issue of piracy, so that’s why I’m recommending this route.