Have you done a PRAM reset, CMD+Option+p+r...
In fact, do 3 in a row, takes a bit of time.
Long shot, but...
Open Audio Midi Setup in Applications>Utilities, see the input & output options & KHz setting there, some things will change it for their own use, then not set it back.
There are a few possibilities.
Make sure there is nothing plugged into the Headphones port. (If there is, unplug it)
Make sure it is not muted. (Press the F10, F11, and F12 keys to adjust volume.)
Check for Software updates
Click the in the upper left
Click Software update. (Installing all available updates is recommended)
Check the System Preferences> Sound> in the 'Output' tab.
Do you see 'Internal Speakers' as shown below?
SMC Reset http://support.apple.com/kb/HT3964