Sound problem

So I have this for like a month now, my sound turns randomly off when I'm just browsing on the computer, or watching vids on YouTube, and when I reboot my computer it works again, but turns randomly off again. So this is what happend before I got this:

I brought my computer to a man who fixed my computer because there was a virus on it, he said my sound card didn't work properly so he removed it and took one of his own cards and putted it in mine. He came back serveral times to see what the problem was, but he kept saying that it was working properly, so he doesn't know what the problem is too. I tried to install other sound drivers, but that didn't work too, I tried Google, but they have no answer for me.

Does anybody know what happend?

 

Next time it happens, check your windows log files to perhaps find a more accurate error to work with.

 

That's not what I'm going to do. I could just buy a new sound card instead... I just want to know what is going on.