I recently rebuilt my Mac and copied out my iTunes folder to an external and back onto the newly rebuilt computer.
I didn't want to migrate my keychain db for more of a "clean slate" experience. Everything works great the only exception was I'm a huge consumer of audible.com audiobooks and they didn't want to sync to my iPhone anymore.
The message I was getting was my computer was not authorized to play these files so they would not sync my iPhone. The solution was easy, all you have to do is double-click on an audible file inside iTunes and it will prompt you to reenter your audible.com or now Amazon.com authentication credentials once that's done everything works.