Before heading over to 1 Degree, there are a few steps you can take in order to remedy the situation:
 

1. If you are using Headphones, try using another pair of Headphones/Earphones to see if the original pair is the problem.
    
2. If the headphones are not the problem, there are some settings you can check in order to try and resolve the problem.
    
   1. Open System Preferences.
       
   2. Go to Sound.
      
       
   3. Make sure that;
       
      1. Your Headphones are the chosen Output.
         
          
      2. The Sound Bar is not at the far left and the Mute button is not ticked.
         
          
3. Restart your computer, if none of the above worked.
    
4. Attempt a PRAM Reset, instructions on how to do so can be found here.
    

If you are still having issues please visit 1 Degree.