Hey, I use GNOME desktop on CentOS 7. When I try to play sound on my user it works fine.

shows soundcard properly.

But I have added new user who is unprivileged to run some program on it. When I did

and tried to play sound then heard silnence instead of expected sound.

executed by new user shows 0 cards.

Then I have added new user to audio, video groups. Then relogged him and it started to work.

As you can see the problem was solved but I really don't understand why? Could you explain to me why? The very first question is why my main user on which I'm logged into desktop has working sound without being part of those groups and new user must be?

pulseaudio is a per-user daemon. So when you su to another user it does not connect to your pulseaudio daemon, it starts its own, but your original daemon is still using the sound hardware, so instead the new user's pulseaudio daemon either sends the sounds to a dummy device and you hear nothing, or the sound producing process blocks waiting for the sound devices to become free.

I decided to disable pulseaudio and go back to using ALSA directly because of this exact reason, and it's the primary reason why I think pulseaudio sucks!

In a word, ownerships, groups and permissions.