Hopefully one of these steps should solve the problem.
1. System Preference > Sound > Output > Internal Speakers
Highlight Internal Speakers / Headphones
Make sure Mute is not enabled.
2. Remove the headphone.
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.
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. Applications > Utilities > Audio MIDI Setup
Audio Devices window
Click the Built-in Output.
Under Mute any of the boxes checked? If so, uncheck it.