I had to open up my Dec 2017 vintage hand crafted Zynthian to verify that it is using an AudioInjector board with a WM8731S and no crystal (no pads for one either) using the AudioInjector driver from the Zynthian setup menu. (I have the original Pisound card, with microphone missing) Their 2 basic cards are $21.92 and $26.72, should you want a reference board. (Just noticed they are WAY cheaper at Amazon with fast free shipping $16.90 and $12.90)
You might check out his AudioInjector test Scripts, just guessing this might be the model
https://github.com/Audio-Injector/stereo-and-zero
(I could not locate compete Audio-Injector schematics.)
There are 2 variations of Pi Zero Audio Injector cards, board pictures show one with a crystal and one without. One article showing a no crystal version (I just recalled Jitter is the term for a shaky clock artifact that a crystal might fix)
I recall having trouble with a different audio board raspberry pi project, I ultimately found the sliders in the Pi audio settings were not right. (I have pretty shallow insights into the working of drivers)