Go step by step and test.
1. System Preference > Sound > Output > Internal Speakers
Highlight Internal Speakers.
Make sure that Mute is not enabled.
2.Plug in and remove the headphone.
If you see red light in the headphone port or not, 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.
Try this even if you don’t see the red light.
3. Reset PRAM. http://support.apple.com/kb/PH14222
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.