Well, then I would look at the possibility that there is some sort of OS or Registry corruption, how to fix this maybe a repair install or a fresh OS install.
If the onboard audio had been disabled in the CMOS setup then the device would not show up in Device Manager.
One other thing you could try to do is to reboot into SAFE mode and then uninstall any AUDIO drivers/apps that relate to this or any other Realtek audio codec and then under Device Manager remove/uninstall anything that also relates to the Realtek audio codec, including any duplicate entries.
Upon rebooting into normal mode, say no to installing any drivers for the audio codec and run the Realtek install app to see if this repairs any registry entries and such and reboot when prompted to.