The speakers on my laptop are going on and off.

I have this weird problem when watching Youtube: Sometimes only one of my laptop-speakers is active and the second one kicks in about 10 seconds later.

The same happens with external speakers (which are brand-new).

Right now, in the middle of a song one of the speakers just went deaf.

But Youtube seems to be the only occasion when this happens. I never noticed that on other websites or when running videogames.
EDIT: I just noticed it also on another music-website.





And the laptop is not old enough for age-problems. It's just maybe 5 years old and should last another 5.

Is it a hardware-problem? A driver-problem?