Update: December 10, 2012 - THIS ISSUE WAS RESOLVED IN ITUNES ON FRIDAY, DECEMBER 7.
iTunes is aware of the issue with preview buttons not working in their store. When you click to play the preview button in the iTunes Music Store, it may play silently for a few seconds and then it will abruptly end. iTunes is working to fix this, but at this time TuneCore cannot speed up the process.
We ask for your patience with this matter. It's not necessary to contact our Customer Care team as we and iTunes are aware of the issue.
We are doing all we can to get this resolved as soon as possible.