CORRECTION: The AMDGPU driver IS installed on my system, which is currently Linux Mint MATE 20. 8-Dale
I do get sound over HDMI now. However, my sound settings are not retained between boots. I have to reset my sounds preferences after each boot. 8-Dale
I had 1 issue. I installed VLC and set the volume low (or off, I don't remember). That also set the global volume that could not be restored.
If your HDMI monitor can decode digital stereo (which is the most basic one) and sound well, but not on other streaming sources, you'll have to trace the audio stream format