I have zynthian working with a behringer uca222 USB interface.
The web interface didn’t work for me, jack is reading the settings from somewhere else.
What you have to do is to modify the file /root/.jackdrc
Mine look like this:
/usr/bin/jackd -P70 -t2000 -dalsa -dhw:CODEC -r48000 -p256 -n4 -s -S -Xraw
Where “CODEC” is the name of the device (uca222), you can run this command to see the name of your device:
$ cat /proc/asound/cards