Update: I don't remember what I did with JRiver volume and Windows volume, but things mostly work. There is still something I don't understand going on inside my computer. I do have a work-around, though. If I have only my pre-pro connected (to the HDMI port), everything works the way I am used to. I still have to select what audio processing I want my pre-pro to perform (Matrix, DTE, none/stereo...), but I get sound. If I connect my Eitr (2-channel device, via USB), then output to the HDMI port goes wonky, and if I try sending a multi-channel signal to the Eitr I get an error from MC28 (the latter is understandable). So if I want to watch Video, my Eitr in unplugged from my computer; if I want to listen to music, I plug in my Eitr's USB cable.
I think I might be able to fix this using Zones and Zone Switches. Before I try constructing my Zones and Zone Switches later this week, I have another question. My Eitr shows up in Windows Control Panel as a Device. However, my pre-pro does not. If I can find out what the ID for my HDMI port is, can I set up a rule to route all Video files to that port, rather than a Device? I think my 2-channel rule is easier: IF [Media Type] = Audio, THEN [Device] = Eitr.
This may be something you guys can't answer, and I understand if it is. I'll experiment anyway.