One other thing I might try is:
Go to Control Panel > Sounds.
Select Digital Output and click on properties.
Select 'Don't use this device' from the drop-down menu under 'Device Usage'.
This should remove the S/PDIF device altogether. Perhaps the speakers will work now?
If not, then you may have to reinstall Realtek drivers in order to return the S/PDIF output to the list of devices.