Try connecting regular headphones (like iPod earbuds) into the headphones port. Do they work? If they do not, System Preferences
Sound pane
Output tab is set correctly, and the sound volume is not set to
mute, there is something wrong with the headphones port. If they do work, there is something wrong with the speakers.
If the problem appears to be the port, you might try resetting PRAM. Some sound settings are stored there.
http://support.apple.com/kb/HT1379