So I upgraded to 18.04 and the sound was gone, as well as all devices from the Output/Input sections (I have an external USB sound card). I tried starting Pulseaudio up in the terminal, but no luck: daemon error. I found the advice somewhere online to try: pavucontrol, and it worked. The sound is back, as well as the devices. I still can't start Pulseaudio up:

E: [pulseaudio] pid.c: Daemon already running.
E: [pulseaudio] main.c: pa_pid_file_create() failed.

How do I make the moment last and have my sound on every startup?

Thanks!

Why do you want to start pulseaudio? The errors you report indicate it's already running.

are you saying you have no sound again after a reboot?

Here's the thing: there was no sound at all after a reboot until I hit "pavucontrol" in the terminal. It brought the sound back, but only for the remaining session - and without Pulseaudio enabled, as there was some kind of daemon error.

I solved the problem halfway by uninstalling the pulseaudio. Now the sound is there after each reboot, but without Pulseaudio it's just the old, same bad Ubuntu sound.

I'm having problems installing and running Pulseaudio back anyway: a daemon error, other errors... it just won't run. What do I do?

The daemon error you report is the error the daemon makes when you try to load it because it's already loaded. Do you mean a different error?

Getting the config files in /etc/pulse right may be your solution.

What's bad about 'Ubuntu' sound? I have only 1 sound, no matter the manager; pulse is just a management system; it doesn't decode audio files or produce sound.

What's the error? The only error I can imagine is that you haven't removed it in the first place and it rejects redundant installation.

What do and report?

ok, but when you reboot again, do you have sound or not?
assuming the answer is no, you know the solution to your oproblem, you just need to make it permanent?

Thank you for your answers. I know that my feedback is a bit chaotic, sorry for that.

1. Yes, at the moment the sound is there after the reboot.
2. After the upgrade to 18.04 the sound quality is not as good, as previously. I think that the reason is that the system has ceased using my external USB sound card as the default device and I can't get it back to work.

The "pamixer --list-sources" command brings up only the following:

Command 'pamixer' not found, did you mean:

command 'amixer' from snap alsa-utils (1.1.2-5)
command 'amixer' from deb alsa-utils
command 'pnmixer' from deb pnmixer

Then that's the problem: you need to make the USB adaptor the default or make the apps use it. My builtin doesn't work reliably so I use a USB adaptor. You can do this in /etc/pulse/default.pa. I do it in /etc/mplayer/mplayer.conf because I use mplayer for all audio.

pamixer is a new package that allows setting the default device for pulse. You don't have to use it but it may be convenient for you. I use Slackware; perhaps Ubuntu doesn't have it yet.

OK, but Pulseaudio tells me that the sink (both actually) is "suspended", reason: "idle". How do I change that?

Oh, and: bash: /etc/pulse/default.pa: Permission denied

don't fix what isn't broken.

about that:
are you sure it's not just volume levels? have you opened pavucontrol or another pulseaudio mixer application to check all levels? it's a little confusing and will take a few minutes the first time.
but yeah, it could alaso be the default sound card. that can, to my knowledge, also be changed in pavucontrol (permanently), or with alsamixer (temporarily; if it helps, we will make it permanent).

What command returns that?

Then $HOME/.config/pulse/default.pa for your personal use.

These are text files that one edits with an editing program, not executables.

This is what happens when I open Alsamixer:

https://ibb.co/iFvToK

These are available devices:

https://ibb.co/b9nxhe

These are the USB device settings:

https://ibb.co/iafiNe

How does it seem to you? How do you make sure that the last device (the USB one) is the default one?

By editing the relevant configuration file.

i really, really don't understand you.
previously you said you HAVE sound after rebooting?
so everything is ok in that department???

anyhow, to literally answer your question i'd start with a web search and take it from there.