sound?

The sound card may be missing, misconnected or dead. It may also not be recognised by the operating system.
The audio drivers may be missing or corrupted.

Shut down the computer and disconnect it. Open the case.
Look to see if you effectively have a sound card or an integrated one. If you don't have one, you can have a decent one for prety cheap, maybe under $20.
Make sure that it's properly connected in it's expantion slot.
Several sound cards will just work right after been installed, other will require you to install some driver.

If you have one, check in the device manager. It may have been disabled. Re-enable it if needed. In this case, it may have beed disabled by accident or some one put a joke on you.

Try to un-install your audio drivers and reinstall them.

It's possible, just after installing Windows, that some devices are not recognised. I had this isue with my integrated ethernet adaptor and sound card.
In this case, let Windows update itself. It will find the appropriate drivers and install them automaticaly. In my case, I had to install a salvaged ethernet card to connect and have the auto-update work...

Electro
November 2012