Hopefully one of these steps should solve the problem.
System Preference > Sound > Output > Internal Speakers
Make sure Mute is not enabled.
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".
If this does not help, contact Apple.