OK, so I rebooted into 4.13.0-39 and as soon as I was logged in the USB audio started working. So I thought maybe that indicated a regression between 4.13 and 4.15. HOWEVER< I then rebooted again, into 4.15 once more, and now I have audio in and out on the USB devices.
I'm not sure exactly why, but for this issue the resolution seems to have been "reboot a whole bunch of times and try a different kernel".
OK, so I rebooted into 4.13.0-39 and as soon as I was logged in the USB audio started working. So I thought maybe that indicated a regression between 4.13 and 4.15. HOWEVER< I then rebooted again, into 4.15 once more, and now I have audio in and out on the USB devices.
I'm not sure exactly why, but for this issue the resolution seems to have been "reboot a whole bunch of times and try a different kernel".