Click Start, choose Run and type in there devmgmt.msc and click ok. That should bring up your device manager.
Do you have anything with an yellow question mark or exclamation point in there?

no yellow ? mark

Go in Control Panel, Sounds and Audio Devices and verify the following:

Volume is maximum and not muted.
Under the Audio tab/Sound playback your sound card is listed.

If this properties are grayed out go to start, choose run and type services.msc
Look for Windows Audio in the list and make sure is Started and Automatic.

If this didn't work
Try to connect a pair of headphones instead of your speakers and verify if there's any sound this way.

If nothing works last thing I can think off go in device manager and double click on your sound card. Click the Drivers tab on the top and choose Roll Back Driver.