[gtomassetti]

Thank you for that information, Darryl.

Correct, the system is not seeing the HD Native card at all. In fact, I sent a system report to Sonnet, and they told me:

"We do see that the chassis is being seen normally, but is connected through several other peripherals.

We also see that there are no PCIe cards being seen in the device, which is unusual."

This is strange, because I have an OWC PCIe SSD in there which used to be the boot drive for my old Mac. I can access that SSD normally by clicking on it in a Finder window, or by clicking its icon on the desktop, and I have a Carbon Copy Cloner task that hourly backs up the internal SSD to the one in the Echo Express, and that task has been running without error.

I'm hoping an HD driver install will fix it, which I will try to do on Saturday.

Regarding your last question, it was actually an upgrade first from 10.13.6 to Mojave, then from Mojave to 10.15.7. In the Mojave install, I had no audio issues at all; system sounds and music played correctly through the Omni. It was in the upgrade from Mojave to Catalina that the HD card has vanished.