Another thing to try:
Go into your system's BIOS settings and check the settings for the on-board audio. (Usually on the Advanced Tab under "Onboard Devices Configuration"
Sometimes there is a setting for "Front Panel Type". You want to be sure that is set to "HD Audio"
If all setting look normal, you might try setting "High Definition Audio to Disabled, then restart the computer (Save&Exit) and let it start all the way into Windows, then restart, go back into Bios, Enable HD Audio, and restart again (Save&Exit).
See if that activates the on board sound chip.