I run Win 7 x64 on a ASrock extreme 3 870 motherboard that supports both SPDIF coax and optical. I tried a coax cable with a HK receiver and set the spdif device as default. The properties for the spdif show the jacks greyed out (even in Realtek HD manager). My speaker outputs work and the jacks show up in color, indicating that they are enabled.
P.S Ofcourse, I get no [digital] sound, and the receiver never detects digital sound. I have ordered an optical cable which should take a couple of days to reach me.
Edit: Added pictures