With the PC powering off, it could signal some type of hardware problelm?
Did you get any hardware diagnostics with the PC that you can run?
One thing to try and it fixes a lot of problems, go to the Device Manager and RIGHT click on the entry for the sound device and then LEFT click "Uninstall". DO NOT check the box to uninstall the drivers
. Restart the PC and when Windows starts it should detect and reinstall the sound.