Sounds like the headphone jack has messed up! What a lot of people forget there is a set of switches within the jack to redirect the audio signal to the speaker output from the headphone. In your case one of the switch element has shorted across the microphone line or the connection between the I/O board to the main logic board has a problem.
[image|2540999]
Here’s the guide to get it out [guide|4392] and you should replace both of these parts
* [product|IF108-011]
* [product|IF108-012]
That should fix things.