I have the same problem with my optical cable to my 5.1 system as far as I can gather this is microsofts end and they need to bring out an update/fix matey hope this answers your question
I had a problem with my 5.1 setup on W10 too, I fixed it by deinstalling the Realtek driver that had been automaticly installed by windows and now I use the normal Windows driver and everything works. Maybe it can help any of you guys.