The internal mic is routed through the front mic input connector and if this connector gets dirty then it will work poorly or go open circuit. Use a cotton bud and WD-40 ( or similar contact cleaner) to clean the two pins next to the headphone jack input. The contact cleaner needs to go right to the back of the connector. Use a toothpick to push the contacts up and down a few times to clear the contacts.
rkp1312, December 2009