It would sure help if we knew more about the computer. Operating system, brand name, model number, etc.

In the meantime, open the Device Manager (Unfortunately, you didn't tell us which operating system you're using but if XP, then RIGHT click on "My Computer" on your desktop, choose "Properties", then click on the 'Hardware' tab, then click on the "Device Manager" button.) When Device Manager is open, click on the + sign next to the "Sound, Video, and Game controllers" listing and you should see a number of devices and one will be your audio device. RIGHT click on that device, choose "Uninstall", then select "Yes/OK" when it asks "Are you sure?". Now restart the computer.. Upon restart, if the sound drivers are already on the computer, then device will be re-recognized by Windows and the drivers will be reinstalled. While it's being re-recognized, you'll see various notices indicating "New Hardware has been found", etc.

After the device is reinstalled, hopefully it works correctly. If not, then you can return the same area of the "Device Manager" and see if the audio device has a name. In addition to the brand name, etc. from above, the device name/driver information could help us find the correct driver for you.

Hope this helps.