No sound on Macbook Air Yesterday I tried to use Apple TV, it connected but I realised that I don't have Airplay so I disconnected the Apple TV. I then transfered some files using a USB and after that I have no sound on Mac. I get a stop sign when trying to change volume. There is no output device listed. I have restarted many times, tried using the headphone port and USB headphones. USB headphones work, but the headphone port for regular headphones wont work at all. I can get sound through USB headphones or if I pair Mac with Apple TV, we get sound on our tv but no picture. I just want sound back on my Mac.

Please can anyone offer any advice or solution?
This discussion in Apple Support Community may shed some light.  First, check to see that Mute is not checked.  You may need to reset the system PRAM.  Here are the steps: (From Apple Support)


  If your Mac doesn’t seem to be remembering your speaker volume,
  display resolution, or the disk it should be using during startup, you
  may need to reset your computer’s PRAM.
  
  Resetting PRAM may change some system settings and preferences. Use
  System Preferences to restore your settings.
  
  Important:   To print these instructions, click the Share button  in
  the Help window, then choose Print. Choose Apple menu > Shut Down.
  
  Locate the following keys on the keyboard: Option, Command (⌘), P, and
  R. You will need to hold these keys down simultaneously in step 4.
  
  Turn on your Mac.
  
  Immediately press and hold the Option-Command-P-R keys. You must press
  this key combination before the gray screen appears.
  
  Continue holding the keys down until your Mac restarts, and you hear
  the startup sound for the second time.
  
  Release the keys.
  
  After resetting your computer’s PRAM, you may need to update your
  speaker volume, display resolution, and startup disk preferences. For
  more information about PRAM, see the Apple Support article About NVRAM
  and PRAM.


If you are nervous about doing so, if you have an Apple Store close by, you may wish to make an appointment with the Genius Bar.