Comment 4 for bug 1354855

Actually after more Googling I've found a solution here:

https://chrisjean.com/fix-for-usb-audio-is-too-loud-and-mutes-at-low-volume-in-ubuntu/

Also follow the link to the Arch forums and make the changes detailed there.

For me, however, I had to set the volume-limit option to 20 rather than 0.01 to get reasonable volume control. Not sure if this is because of changes in the latest version of PA/ALSA or not, though. Best to start with their recommended 0.01 option in case you blow your ears off.