Hello Micha
It’s ok with the opi zero 3 tree , the HDMI have sound !!!
With your help can we rebuild the dtb file to expand to the jack output
i try with the alsamixer but how to do mith 
too late tonight , to be continued
I’m very happy to have sound on HDMi under Dietpi
This morning i re-read your link from Armbian and remember that i see in alsa mixer the Dacl and Dacr put to “off”
With this
Perhaps a solution for the jack 3,5 mm
I try tonight