No active mixer devices available

Hey everyone.. So, let's see.. I just purchased this Dell Optiplex Gx260 and installed it yesterday. So far so good, but once I downloaded a few things like mozilla, google chrome and other things, my audio device disappeared. Now, when I go into the audio device properties, it says its still there and working properly.. Yet it isn't.. Any way I can fix this?

p.s. I'm not very computer savvy, so if you could make it simple, I'd appreciate it.. :)
 
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