Possibilities -
1.In WMP,go into options/file types and "select all"
2.Double click on the volume icon in the system tray and
check all volumes are at least half way up,and none are muted.
3.Go into Device Manager and check all sound devices are working properly.
4.Uninstall/reinstall sound card drivers.
5.Say that you`ve tried all these,and it still doesn`t work.