Suggest verifying you can play-back the files on the card while it's still in the camera. This will confirm the files' integrity is not in question.
If you can play them on the camera:
Eliminate the transfer process as the issue - if using a card-reader, change the card-reader or use a direct copy from the camera itself using the PC-to-camera cable. If using the camera-to-PC cable, use a card-reader. Are the results the same?