Disable all sound permanently

All right, so blacklisting the sound related modules worked. Thanks to this post:

http://www.pc-freak.net/blog/disabling-sound-kernel-modules-debian-ubuntu-gnu-linux-servers/

The details of the fix:

edit /etc/modprobe.d/snd-blacklist.conf and add these entries (from a Dell standalone pc with 18.04)

blacklist soundcore
blacklist snd
blacklist snd_pcm
blacklist snd_hda_codec_hdmi
blacklist snd_hda_codec_realtek
blacklist snd_hda_codec_generic
blacklist snd_hda_intel
blacklist snd_hda_codec
blacklist snd_hda_core
blacklist snd_hwdep
blacklist snd_timer

A hardware solution: insert a mini-jack connector in the sound output port, but only a connector, without any wire or speaker on it. This has worked for me since the 80's to silence the Mac's otherwise beautiful power-on ding.

A simple solution would be removing the alsa drivers, i.e.:

sudo apt-get remove --purge alsa