I solved it another way, or at least I think. System has been stable in games with multiple monitors and audio is normal.
I used the 19.11.3 drivers with the 20.3.1 Audio drivers. However, instead of the normal 5.1 speaker setup I now choose Dolby Atmos for Home Theater, this requires you to download the app from the store (you don't need to purchase anything unless you want to use headphones). The good thing about this is that I can configure it to have 5.1 or 7.1 or something else instead of the stereo (which still did not work even with pass-through). This solution works with pass-through for me, my receiver now correctly detects Dolby Atmos, DTS, Dolby Digital, that TrueHD format etc.
This seems to have solved it for me; I will report in if things crash 
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I just switched from 5.1 to "dolby atmos for home theater" in Windows, without your split driver solution, and so far the sound seems to work without drop outs. I'm on the newest drivers 20.4.1.
Edit: Nope it didn't 
Do note that on my system after a boot I had to set it again on the Dolby Atmos for Home Theater. If I don't it is stuck on 7.1. So if it was after a reboot that it stopped working check that! I don't have the solution yet for keeping it on Dolby Atmos after a boot .
No problems yesterday while gaming, even multiple screens on and video playing in one of them. Before that I couldn't do that without weird audio drops or screen issues.
Edit:
Nope, unless my config is wrong somewhere it didn't work. Had after 5 hours artifacts and no audio, switching on receiver on/off solves it for a few hours though. It might be better than before but it is hard to tell.
Conclusion, the AMD HDMI driver sucks.
It looks like I've found a workaround for missing HDMI audio problem, which works in almost every game and application I've tried. In fact, there is only one game that still have audio cut-outs, but they are quite rare and the game is quite old itself, but with workaround they are much less frequent. The still-not-properly-working game is Syberia 2 btw.
So, workaround is quite simple: you need to enable "Instant Replay" function
It's in the same place where Metrics Overlay settings are.
The default capture resolution is the same as game resolution, which can be too much for some games and cause slowdowns and stutters. But lowering duration time and bitrate helps, and overall this feature doesn't seem to be very demanding. With 720p resolution capture, HEVC encoding and bitrate aroung 5 Mb/s I still have locked 60 at 1440p and good thermals with low cooler noise.
And it looks like even VLC media player now works without issues with this fix.
My idea is that audio cut-outs somehow depend on gpu or memory utilisation, because in "heavier" titles they are much less frequent. And this is probably why in older games like Syberia issue is still present with this fix - game can't stress gpu enough even with screen capture, because resolution is only 800*600.
And it's probably worth noticing that I use 20.3.1 drivers with High Definition Audio Drivers version 10.0.1.6, also PCIe is manually swithced to Gen4 in BIOS.
Yes, it is definitely adding some GPU load. With my current settings (720p capture, 15 sec duration, HEVC and 5 Mb/s bitrate) it's around 1-2% of additional GPU utilisation. I actually run several benchmarks in Metro Exodus (1440p, Ultra) to measure it, and result are 57.1 fps without Instant Replay and 56.3 fps with Instant Replay. Which is a tolerable sacrifice for a working audio IMO 
Metro Exodus is the most gpu-depending game I have installed right now, and I assume it's a representable "worst-case scenario" for Instant Replay. If you have performance issues with this setting being enable, I suggest lowering capture bitrate and resolution.
And for some reason no other solution, like switching to microsoft drivers or something else worked in my case. The only thing that also works for me is a Metrics Overlay, but it's quite distracting and tend to disappear. So, for some people I hope this will be of use.
