Hello.
There is something strange happening with my sound on Ubuntu. When I turn on youtube video I can hear the sound for a few seconds and then it fades away. When I try alsamixer and enable/disable auto-mute I am able to hear the sound again for a short time. Same thing with reboot or I cannot see anything special in dmesg.

Please help with any tips.

Ubuntu uses Pulse Audio, which sits atop ALSA, as it were.

Install the Pulse Audio Mixer (pavucontrol--it's in the repos). It give you more granular control of input and output streams and devices and should be helpful in troubleshooting this.

You could also try starting your browser from the command line and playing a Youtube video. Perhaps some useful error messages will be thrown to the terminal.

It would be helpful to narrow this down as much as possible. Does this happen to audio from other sources and audio players, either streamed or local?

Thank you for the reply! This doesn't happen often, so I will have to wait for this bug again. I had pavucontrol before and it didn't help.
These are both good ideas to try different sources and start from cmd. I was thinking that source does not matter, but I will definitely try it.