Pixel 2 supports audio adapters and headsets that communicate digitally over USB-C as defined by USB Audio Class 1/2/3. Pixel 2 does not support an analog audio signal over USB-C and subsequently will display an unsupported notification warning when an analog audio device is attached.
Here's the spec documents from USB.org:
So it got me thinking, what kinds of USB audio devices are actually supported?
Audio mixers, like this one from Behringer? Or does it only support USB output, not input? I have such a mixer, but I don't have a Pixel 2 to test.
Car stereos, to my knowledge, have never supported any USB Audio Device class. I hope this changes, but all I've seen is support for USB mass storage mode (only old versions of Android), then MTP (only plays formats that the car stereo supports; no FLAC, maybe no AAC), and then AOA v2, which seems to have been abandoned for some reason (Pixel 2 doesn't support it, nor does the Moto X4).
USB headphones: some recommended models are listed on the Google post I linked above. Are there any that you've found not to work?
USB DACs/amps: here's a thread, they seem to be well supported.
Speaker docks: they only support iPhones, don't they? Has any manufacturer chosen to make one for the Pixel 2, or are there any that support a class of USB Audio?