Thanks for the additional details. I am using SteamVR with its built-in standard overlay containing P2A that I can "float" inside the plane. As my setup is different than yours I probably can't help... other than mention what I have noticed with my setup.
I was having a similar issue the last several days where I would put P2A in the overlay and then when I attempted (using controller) to set the SND devices in P2A Settings, I could not see the drop down list in the overlay, but it did appear on the 2D desktop. I could set the values I wanted with the mouse in 2D. Sounds similar to your issue.
And so I reinstalled just now version 0.0.27N of the EarlyAccess P2A updates... and now it works. So, I have no explanation, other than the reinstall. I can set the values using controller in both a desktop overlay and if I float the P2A overlay.
I am not very familiar with the story on the HP Reverb G2 with WMR (I assume this is Windows Mixed Reality), but as I understand it updating Windows 11 to the latest version might make WMR obsolete. Perhaps you or others who use HP Reverb may know more about that.
Best,
EagleSight