I found the docs on how Qubes gets audio from and to VMs. https://www.qubes-os.org/doc/audio-virtualization/ and I have a pacat-simple-vchan process running (in dom0) for each vm, I have the UNIX sockets in dom0 as well, so everything looks reasonable there. In the VMs, I can see pulseaudio running, and audio played in a VM comes out the speakers, so audio transmission seems to be working fine in that direction...

I've given up for now and am using my Acusis Echo-Cancelling Microphone Array, which is a USB device that passes through just fine (just like all my other USB devices).