I would have to say that it is a new program or game you recently installed that is causing the problem, and uninstalling it should fix it. But, it also could have changed your sound settings, and uninstalling it will still leave the settings the same.

The best thing to do is to uninstall any recent software you installed, and if the sound still doesn't work after that, go to your sound settings and change it to the correct device. To do this, open your control panel and click the sounds and audio devices, then click the "Audio" tab when the box opens.

Under that tab, you should see your Default playback device, which should be the correct device you have that plays your sound. If it is not, or there is more then one selection to choose from, experiment a little bit choosing a different device until the sound works.

You also should check to see if the speakers are plugged into the correct jack port hole on the back of your computer.