I have been trying to resolve the missing hdmi audio when the vc4 overlay is loaded.
I noted a number of config files which are different between the RPiOS and ArchArm in /usr/share/alsa. I noted some stuff here:
viewtopic.php?f=57&t=15917
I am still trying to get through it all, but one file which is not present is /usr/share/alsa/init/bcm2835 which seems like it may be helpful on the RPi. Another which I have noted is: /usr/share/alsa/cards/vc4-hdmi.conf. Also in this directory are some other *.confs which RpiOS has but ArchArm does not.
The archive attached has the files which are different. I have not yet gone through all of them to see which are absolutely necessary but these are the ones which differ in size or are not present on the ArchArm image.