The driver depends on the device and the model of your computer or motherboard. You could provide this information to us, or you could go to the computer manufacturer's site, find the Support section, enter your model, select Download, look for Drivers, and there they will be.
If you have an add-in card, you would get it from the card manufacturer's site.
Try this first: Go into the Device Manager (Start - My Computer and right click on Properties - Hardware tab - click on Device Mangler. Look for anything with a yellow exclamation point or a red x next to it. If there is such a thing in the Sound category, sometimes just removing the device (right-click, uninstall) and rebooting will solve the problem.