If you have had trouble with your internet connection and this has caused you to receive a damaged track file, you will need to remove the downloaded file from your device by either:
Pressing and holding on the track until the menu at the top of the screen appears, then tap the trash icon. Tap "Delete" to confirm you wish to remove the track.
The download icon will appear next to the track that was just deleted. Tap on the download icon to start downloading again and the track should now play.
If it continues to not work, contact customer services at http://7dig.tl/ContactUs and let us know the album or track in which this issue is occurring.