I had no problem getting the 3.5mm jack working. I followed the Adafruit setup instructions until I got aplay outputting sound (I used the sample sounds in /usr/share/sounds/alsa for testing; I needed to adjust the headphones level in alsamixer for this). Then I adjusted the mycroft system settings (sudo $(which mycroft-config) edit system) to set the output device to plughw:CARD=seeed2micvoicec,DEV=0.
The issues I’ve got currently are that the sound levels don’t seem to be saved on reboot, and the wakeword is not picking up half the time I say it, but watching youtube videos keeps triggering it.