I am
not technical enough to tell you what files to compare. But I've
heard of "asound.rc" or maybe it is "asoundrc" (don't remember
sorry), so you could hunt google for that, and Ubuntu/Debian, and
compare the files between the two OSs. But you have NOT taken
advantage of this to do more research yourself yet.
Well, I did. I have not mentionned it (but maybe I should have)
because I did not want to make my post even longer and because it
should work without these files. I checked for existence of any
"*asound*" files in my Ubuntu install: there is no. In debian, none
either. So I thought that’s not somewhere to search further.
The troubleshooting guide for alsa mentions that these files should
not be used in a working install (see Warning here:
http://alsa.opensrc.org/.asoundrc): "If your system won't work
without [these files], and you are running the most current version
of ALSA, you probably should file a bug report."
What you didn't quote is the first sentence of the warning:
"Neither .asoundrc or /etc/asound.conf is normally required."
'Normally' here means (and I know this for certain) 'having a very
uncomplicated system'. It means one and only one sound device, and I
suspect that even then, it means on-board sound or a SoundBlaster.
My server had on-board sound, which usually got designated 'card0', and
I used an external USB device, usually 'card1'. This most certainly did
require a configuration file. I would expect your computer to have at
least two sound devices, and if the HDMI one is being identified as
something other than card0, you will need to specify what device you
actually use.
Note that a different OS running on the same hardware may designate
sound devices differently, and indeed my old server would sometimes
reverse the card order after a reboot, just to give me something to do.