System Preference > Sound > Output > Internal Speakers
Make sure Mute is not enabled.
If you see red light in the headphone port, plug the headphone
in and out of the port 5 or 6 times to flip the microswitch inside.
Sound output may be stuck in the digital mode.
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".