Can you believe I actually have a suggestion on a technology problem?
This used to happen to me a lot, and it used to drive me crazy. I would run every diagnostic test I had, and I would ultimately just reinstall my sound drivers again and again.
Then I found an easier solution to my problem. It was some quirk in the speaker system. When it happens now, I just disconnect a single wire, then plug it in again. It works every time. I don't know if that helps you, but it is something you might try.
If you have some other speakers, plug them into your sound card and see if they work. If they don't (as the others did not) then you know the problem is the sound card. If they do work, then you know the problem is your speakers.
Most often it is the speaker wires as someone else mentioned.