Remove a couple screws and pull the headphone jack out of the rear case.

As with most portable devices, the headphone jack is very well fastened to the case.

The microphone's right next to the headphone jack and goes through the same data cable as the headphone jack.

Like the iPhone, it appears that the iPad features a liquid sensor in its headphone jack. If you get your iPad wet, don't expect Apple to fix it under warranty for you.