If a music store file is corrupted, or if it accidentally downloads the preview instead of the final file, the solution is the same!
- ensure your windows are properly Arranged
- select the bad song files in kJams
- in the Edit menu, pick "Trash original files"
- context-click the music store playlist
- pick "Sync Previous Purchases"
- watch the Activity window and wait for the downloads to complete
Your songs should now play correctly! If they don't, please let me know the store name, and the song details (name, artist, album), and I'll investigate