It's probably a driver issue.
Access device manager by:
Windows XP: right-click My Computer>properties>hardware tab>device manager
Windows 7: right-click My Computer>properties>device manager
Look for the sound and if it has a yellow triangle with an ! inside it. If so that means the driver isn't loaded properly. You can try to right-click and choose update driver. You can let Windows search for one. If it doesn't find it, go to Dell's website and go to the support section and you can enter your service tag (or let it find it for you automatically) and it should give you a link to re-download it.