I have a problem the same but it was on a computer with XP on it.
I went to the control panel and clickeed on "Sounds and Audio devices" and looked at the Audio tab to see what the sound was set to play through in that and found it had changed. I had two different things listed in the drop down box, I changed them and it worked for me. Hope this is some sort of help for you.
You could also check in your Device manager to make sure that there is no symbel next to your sound card to say its not working.