Hi! This module is now included in the 4.19 kernel recently uploaded to experimental (linux-image-4.19.0-trunk-arm64 4.19.5-1~exp1), you can add experimental to your sources.list like this:
deb http://deb.debian.org/debian ../project/experimental main However on my first tests this hasn't solved the problem, looks like the driver is not finding the hardware or similar, I'm getting: snd_bcm2835: module is from the staging directory, the quality is unknown, you have been warned. but not the: bcm2835_alsa bcm2835_alsa: card created with 8 channels message, so it doesn't work. We know that the current dtb is broken since at lease 4.18, this breaks wifi and it might affect the sound so that it is not found, but I don't really know. If you test it and get it to work just let us know how you managed it. Regards. -- Manty/BestiaTester -> http://manty.net