Some users are finding that after updating to iTunes 10.1, their music is playing entirely in mono sound. Though iTunes itself does not have the ability to change your machine's output to mono, the Audio MIDI Setup utility does.
As Apple Support Discussions user Roger Wilmut1 suggests:
Check this: go to Applications / Utilities / Audio-MIDI Setup. Click on "Configure speakers." In the pane which opens (right) check that the speakers are indeed set to left front and right front. Click each speaker to check the sound. Are you using external speakers? If it's still in mono check your connection: if it's a 3.5mm plug is it inserted fully?"
The original poster checked his connections along with playing the same files in QuickTime, Logic Pro, and via Finder's preview function--all worked fine. After reverting back to iTunes 9.2, the problem was solved.
Of course, this does not answer why the files may have been playing as mono in iTunes 10.1. It is possible that a third-party plug-in is the culprit. Users should be sure that all their software is up-to-date at all times, especially when major updates to large programs like iTunes are released.
Often, it takes third-party vendors some time to update after major releases. If you find that some of your plug-ins are not compatible with the latest version of iTunes, check with the developer for a new release or delete them. Test your issue again and chances are, you'll be good to go.
Do you have any issues with iTunes 10.1? Let us know in the comments.