Comment 2 for bug 1790816

It's a shot in the dark, but in my case, pulse sound server (atop of alsa) seems to have gotten into a mess with an upgrade between 16.04 and 18.04. Post login, removing old / session start user config fixed my dodgy channel mappings (but not quite the same as crackling, etc):

pulseaudio --kill \
  && rm -r ~/.config/pulse/* \
  && pulseaudio --start

However, every new boot, the same missing channel mappings cause my audio to sound bad (not mapped correctly to 5.1 channels).

Also, try alsa-info which is a script that bundles all the alsa inspection commands and then maybe try fish for the problem in that. Also try figure out how to play a sound file directly to alsa vs via the pulse sound server to rule out pulse causing the problem.

I think the speaker-test command also directly tests the alsa layer where as the gnome-control-centre (settings) sound page "Test Speakers" tests the setup via pulse.