In place of the headphone jack, we find a component that seems to channel sound from outside the phone into the microphone... or from the Taptic Engine out.
Teardown Update: According to Apple, this plastic component is a barometric vent. With the added ingress protection afforded by the watertight seal, the iPhone uses this baffle to equalize the internal and atmospheric pressures in order to have an accurate altimeter.
Mechanical buttons are a thing of the past when you have a Taptic Engine! This sleek engine uses haptic feedback to simulate the push of a button, without having a real button.