Is anything at all showing in Device Manager under Sound Video and Game controllers? Anything for Realtek or Cmedia etc?
If there is, uninstall it and restart your PC.
If there is't, try going into Smart Restore, click Hardware, Media, Install.
If all else fails, try running a recovery, if the recovery doesn't work, do you have a Coverplan or PCPerformance on the PC? If so call: 0870 901 3000 to get your mobo swapped, if not...Go to a PC shop and pick up a sound card...Usually around £10.00 for a basic one.
When you fit it, don't forget to drop to BIOS to turn off the on-board sound chip.