Today, utterly randomly and with no provocation, the built-in speaker output disappeared from my System Preferences on my MacBook Pro and the speakers would no longer work, except for the boot-up chime. All that I could get was digital audio out and the red light coming out of the speaker port. Searching online turned up nothing until the right combination of words led me to this Apple support message board thread. While Apple does not have a solution for this problem and tries replacing the board with the audio port, you can solve the problem by sticking a toothpick or a paperclip into the slot (preferably while the red digital audio light is on) and at around the 5-o-clock position there is some sort of switch or rocker that needs to be gently pushed. There will be a bit of a click. After a few seconds your audio will start working again.