Several users have reported an issue in which some iTunes-purchased music and video files in the .M4P format will not play properly after the update to iTunes 9.1. In these cases, the files will either not play altogether, or exhibit choppy, broken playback.
The problem seems to primarily affect older, FairPlay 2 files.
The only fix for this issue we have discovered so far is less than ideal and involves downgrading to iTunes 9.0. To do so, completely remove iTunes per these instructions then download and install iTunes 9.0.
Feedback? info@appletoolbox.com.
April 22, 2010 at 1:12 PM
I had this problem and got a total runaround from Apple’s tech support. I would try to play a DRMed iTunes song–and a dialogue box would pop up, telling me that my computer wasn’t authorized to play that particular song. It asked me to provide my iTunes account name and password. When I did, iTunes would tell me that an error occurred in the iTunes store. I still was not able to play the song.
I finally figured out that if I simply quit iTunes and then re-started it, the app would play my DRMed iTunes songs.
In some cases, you may have to restart your computer.