The files maybe corrupt/fake and you'll need to keep trying. See:
Recognising Fake files and also:
How to find music
If your using the LW player, disable it and use your default one. The LW player is known to act up and is not very reliable. In LW go to Tools>Options>Player, untick the box and click apply. Now files should launch in you default player.
If your already using your default player then it might be a codec issue. Try using a different player.
VLC media player has the codecs built in and is very reliable. If it plays in that one then you know it's not the files but the player that's causing it.
See:
CODECs list for Movies/Alternate players / Video help forums/ video-mp3 fixers