If red light is seen through the headphone socket, plug the headphone into and out of the socket
couple of times to flip the micro switch to turn off the digital sound.
Reset PRAM http://support.apple.com/kb/PH4405
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".