1 System Preference > Sound > Output > Output Volume
Make sure that the box Mute is not checked.
2. If red light is visible in the headphone port, plug the headphone into and out of the port
to flip the microswitch inside the port. Sound may be stuck in the digital mode.
3. Reset PRAM. http://support.apple.com/kb/PH4405
4. Reset SMC. http://support.apple.com/kb/HT3964
Choose the method for:
"Resetting SMC on portables with a battery you should not remove on your own".
5. If these steps do not work, plug in the headphone in and reset SMC.
6. Apple store.