So I let windows update drivers and I had discord and steam turned on while running table top simulator.
Then I ran over my cord with my chair, and I unplugged the vr set, massaged the cord and plugged it back in, and it still wasn't working so I uninstalled and reinstalled WMR portal, and then on the reinstall it said it couldn't until I was up to date with my windows updates.
So I installed my windows updates. And then it happened. I couldn't get Audio mirroring working.
The initial behavior was I would click the toggle in the settings to turn it on, and then the next time I visited settings it would be off again. And it didn't help or change the state at all.
I think I also uninstalled SteamVR in this mix, too, trying to fix it.
So lets get down to it. The internet says I can fix it by setting this regedit value to 1:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\HoloSI\Audio
AudioMirroringEnabled=1 (DWORD)
So my normal windows user didn't have permissions to modify that part of the registry.
My local admin user didn't have WMR installed, and the setting didn't even show up when I launched Settings.
So I ended up going into regedit as an admin and going to that path and changing the permissions for the path to allow my non-admin user to change it. Full-access for Users. Done.
I then did an uninstall and reinstall of the WMR Portal.
Now the setting was showing that it could change. I also got a nearby setting called KnownHmdEndpoints that got populated next to the AudioMirroringEnabled.
But it still wasn't doing anything. No change in actual sound behavior.
Finally after a lot of guessing and trying all sorts of other garbage, the final fix was to go into SteamVR audio settings and enable the mirror there.
Hopefully this post is useful to someone on the internet someday.