Well, I had this problem last night, wondered in forums to find a solution but was unsuccessful (Even replacing all the .plists in ~/Library/Preferences by the ones of a new account), the only way I could get it back was doing the following:
Created new account, tested it and the previews there appeared, so I opened /Users/(Name old Account) and changed the permissions of the entire folder & contents to (staff=Read / Write), then manually backed up all the plists in ~/preferences, the Application Support folder, Safari and Keychains, and the folders where I had files like Pictures, Documents, Downloads, Desktop, Dropbox (I did copy them because I hadn't have many things on them, if I changed the permissions I guess it would let me just move them, which would be faster), so, next step was deleting my account from System Preferences and recreate it after, login to it again and put all folders back to their places, repair permissions and done. My account was again the way it was, with mp3 icon previews!
I know it isn't a good solution, but beats the crap out of formatting again to solve this!
