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.