You have most likely muted your sound output in your sound card's mixer panel, perhaps for the CD-ROM drive; in another program that uses sound or even the mute facility in WMP itself.
It's very easily done but sometimes hard to fathom out what has gone wrong..:-)
I recently switched to a SoundBlaster Live! sound card from the (very good) onboard C-Media 8738 soundchip and discovered I had two sound tracks running simultaneously.
I eventually realised I would have to mute the CD-ROM's audio output in the sound mixer to cure it.
Yes, before anyone jumps in, I had already disabled the onboard sound in the BIOS set up...:-)