My reading around has suggested something about a switch being inadvertently tripped. I am trying to plug and unplug as with my headphone jack, as suggested by a couple readers, but I am wondering, what sort of input, digital or otherwise, would there be through the audio output jack when this is not one of the integrated jacks, with both headphone and microphone together? This has a separate port for the microphone.
Might I be looking at the wrong answer?
The Airport Express uses this kind of jack also for the possibility to plug an adapter in and transmit the audio signal digitally via a fiber optical cable. This system is called TOSLINK.
Some MacBooks that are not too old have this technology. The great thing is, that you have one jack for two possibilities of connection one digital and one analoge. http://support.apple.com/kb/HT1596?viewlocale=en_US
So as far as I understand it, your MacBook doen't want to report any fault by showing a red light in the jack, it only has technological possibilities you didn't know.
I recently got a new MacBook Pro, and I decided to take the data off my old MacBook. I ended up freeing up 300 of the 500 Gb on the HDD. In so doing, it also apparently freed up whatever adjustment was made, allowing for the sound to once again come through the speakers.
I would therefore suggest to people to check their HDD usage levels. I was at about the 85-90% level, and I am now thinking that that may have been what caused the loss of connection to the built-in speakers.
Just my $0.02.