Hopefully one of these steps should solve the problem.
1. System Preference > Sound > Output > Internal Speakers
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.
Option click the speaker icon, if present and select Internal Speakers.
I tried all those things to no avail. But I did find a solution. I plugged in a 1/8" jack into the headphone port, then I went to System Preferences > Sound. There, under Output, I selected "Internal Speakers" and changed "Use audio port for" to Sound Input (not Output). Now I can hear my laptop speakers. If I remove the 1/8" jack (or headphones), the sound disappears from my laptop speakers. But at least I can hear the sound now from the macbook pro internal speakers.
My solution was that I had left my "SYSTEM PREFERENCES" - "SOUND" set to Apple Tv.
Suggestion to Apple management.
When the Mac is displaying a screen that has sound, ask if sound should be turned on if it is not.
When the Mac is displaying on the Apple Tv a screen that has sound, ask if sound should be turned on if it is not.
When the Mac is sending sound through AirPlay, ask if sound should be turned on if it is not.
If the Mac is doing somthing with sound. Enable both the original and the internal speakers when the speaker icon or keys are adjusted.