Sound But NO SOUND

I have a P4 PC with GigaByte i845 chipset w/ built-in sound mainboard. I installed Windows 2000 and installed all the drivers. Everything seems fine and I can also see the “speaker” icon on lower-right corner of taskbar, which means that the sound drivers are installed properly. But the problem is that I can not hear any sound from the speakers. The .wav, .mp3, .mid and others type of files do not give any sound to speaker however I can play and listen to the Audio CDs.
What could the problem be?
For time being I disbled the built-in sound and installed add-on sound card, it works fine with all types of sound files and multimedia applications.