um......go to your hardware profile, find out what you have for a sound device, and then download the driver
if it comes up as unknown device
use windows update, to find out what it is, then download the driver
ah okay it said legacy audio drivers and then i went to troubleshoot and it took me through a whole load of options..are your speakers on etc. I dont even have the little speaker icon at the bottom right hand corner of my screen.
Try this, right click on My Computer (Computer in Vista), then click Manage. When the new screen comes up click Device Manager. If your audio driver is missing, it should say something like audio controller with a yellow exclamation symbol next to it, right click that and then click Update Driver Software or something and tell choose the option for windows to search for the driver online, should take a little while. I've done this on one of my computers and it's worked before.
Find out what sound card you have, and google it. Download the driver and install it like you would any thing else.