Nice work !
I am aware of this problem, have a look at this post here : Audio injector Isolated card needs modprobe for Linux version 4.19.xxx
Until Volumio starts using the current Rasperry Pi kernel (version 5.x.xx) the following has to be run after booting :
sudo modprobe snd_soc_cs4271_i2c
Alternatively you can create or edit the /etc/modules file and add the following line 
snd_soc_cs4271_i2c