Hi, have you had something plugged into the Audio Out port late;ly? Seems the jack don't close properly often enough, sometimes carefully using a wooden toothpic to root around in there fixes it.
Long shot, but...
Open Audio Midi Setup in Applications>Utilities, see the input & output options & KHz setting there, some things will change it for their own use, then not set it back
I had this same problem. If the toothpick or paper clip doesn't work, you may have to send it back to apple. Find an app,e store near you and take your computer in. You can have them send it back to the manufacturer and repair your output. This is a common problem amongst Mac users it appears. When you send in your computer, they will also repair anything else they find wrong with your computer for a down payment of around $500 total. I had to do that with my laptop to get my graphics card replaced. Hope this helped! :)