Hi,
1. Do you get sound in Windows Media Player if you try to play audio songs or MP3?
2. Is the issue confined to particular video files?
3. What is the format of the file you are trying to play in Windows Media Player?
Method 1:
Open the Windows Media Player Settings troubleshooter
http://windows.microsoft.com/en-US/windows7/Open-the-Windows-Media-Player-Settings-troubleshooter
Method 2:
Change volume settings in Windows Media Player
http://windows.microsoft.com/en-US/windows7/Change-volume-settings-in-Windows-Media-Player