Big update here Jeff and in the end it turns out I really am an idiot. My developers sorted me out and the answer is insultingly simple, as we all probably knew it was going to be. The thing that brought it to light was moving my testing into 5.1
When I tried doing previews in 5.1 I could get all the sounds working in Selected Viewport preview on windows. When I did that, all the audio also played off the headset for some reason. Both native and FMOD. Nothing at all would play from the headset when I ran VR Preview.
Then they said “do you have Oculus chosen as the audio output in Windows.” Well no, I didn’t. It never occurred to me that this was necessary because I always had some audio playing out of the headset in spite of this, and assumed the editor handled all that routing. When I changed this I had the expected result of all audio working on the headset.
So I went back to 4.27 and this made all the audio work as well.
So a true idiot’s guide here would probably have sorted it out. But there is not one place I’ve found that explains the nuances here of why I was getting some audio from the headset despite leaving windows sending audio through my USB interface.
I don’t pretend to understand that but maybe you do. There’s probably some documentation ramifications here for future idiots. Anyway, thanks for the help.