I’ve downloaded Roon Bridge on my Windows HTPC, and I can report that it works correctly using the 7.1 layout with a 5.1 file. That is, the back channels are properly routed to my side channels. HDMI is the default output on that PC, so I set it up to use the default device. It works the same if I select Intel Display Audio as the output. But stereo files will not play.
If I change the channel layout to 2.0 or 5.1 on the Windows machine neither stereo nor multichannel files will play. In fact, nothing will play. It seems to try and then shows nothing is playing.
Frustrating.