Hello,
just to share my experience with 5.1 in windows 10.
at first im also having issue where i only get 2 speakers working even my sub is not working.
so what I did was to go to my Motherboard's support page and downloaded the latest drivers for my realtek builtin soundcard. and then a "Realtek Audio Console" App was installed and did the setup there.
then i found this option "Speaker Fill creates a virtual......." make sure you enable it. and do a PC restart.
then after doing all this are are working great. even Spotify is surround now.
just that the videos/music from browser is still only on stereo. but Netflix, VLC, Games are all surround.
hope this helps.