When I used dom0 as audio-vm, I could plug in my HDMI-monitor and play audio via the monitor. All I had to do was to select the correct profile in the “Configuration”-Tab of pavucontrol.
However, my sys-audio-VM doesn’t seem to be capable to output via HDMI, at least the profile is missing. I’m using a minimal template (fedora-38) for sys-audio, so possibly I’m missing some drivers/packages? on the other hand, maybe the HDMI-port is part of the (intel integrated) GPU and inaccessible to sys-audio? Then again it’s possible it’s a bug.