Well, guys. I've read through all the posts and decided to help those who have the same issue and setup like mine. As for the latter, I have Windows 10 Pro x64 with Dolby Access installed, my PC is connected via NVidia Display Port to HDMI port in Onkyo
TX-NR575E (Dolby Atmos enabled) AV receiver (Not sure if it's important but CEC and ARC are both enabled in Onkyo).
So when I first installed Windows I could easily select "Dolby Atmos for home theater" option and perfectly play Dolby Atmos content (via Dolby Digital Plus stream in my case), until some days ago, when it became unselectable. Well as most of you I spend a
lot of time trying to figure it out, but then I noticed, that my receiver is connected via NVidia High Definition device, not Intel one (as for most posters here). So that was my lucky guess.
What I did next has solved the issue:
I manually deleted all NVidia audio devices in Device Manager (no reboot);
Having Nvidia 415.xx Driver installed I've downloaded 399.24 driver (just selected the one at the bottom of the list);
Installed 399.24 driver, selecting Custom Install and checking "Clean Install" checkbox (reboot);
After that I checked the Sound settings, but I was still unable to select "Dolby Atmos for home theater" option.
So I uninstalled Dolby Access app, reinstalled it and launched it. At this point the app asked me to reboot which I did.
After boot, I was able to select "Dolby Atmos for home theater" AGAIN! And play Atmos content of course! ))
Not sure you want to rollback to older driver, but I just described what I did to solve the issue, and all you need is basically the clean install Nvidia drivers, as well as reinstalling Dolby Access app. That's it.
Hope it will help.
P.S. Despite the fact that my Onkyo is Atmos enabled and capable (being connected to XBOX One X it's showing Dolby Atmos on it's screen and in input info), in Advanced sound settings on my PC I don't have Dolby Atmos on Supported Formats tab...