Sound suddenly gets quieter on the laptop, why?

I have googled it for the third time now because it has happened many times, but I do not know any more advice. I had been on the laptop for a while, until suddenly the sound dropped from 100% to perhaps 2-3%, without changing the system volume itself.
In a few words: the sound is at 100 according to the display, but effectively gone.

! But it works normally with headphones!
-> It's an HP Pavilion

I have already tried:
in the sound menu to fix the problem
to remove the rights for the respective sound output
to reinstall the driver
down the laptop several times and start up
to run the virus scan

and to check if it can be anything else… What else can I do?

What do you have to do for the sound to come back?

Which programs are running at the moment when the sound goes away?