When your computer starts up, press the key to enter BIOS, find and enabled the onboard audio. The HDMI audio for your graphic card would of disabled this and possibly enabled HDMI audio instead (disable that if there and not wanted).
Start up Windows. Right click on the white speaker in your system tray, select Playback devices. Set the default to the original sound card.
If that didn't solve your problem or you wish to clean up and remove:
Uninstall the HDMI drivers from the Control Panel > Sound and Audio Devices and re-install the old sound card drivers if necessary.