When will I hear the sonic boom from a plane flying towards me?
In case a supersonic aircraft (flying faster than the speed of sound) flies directly over a person, that person (the observer) will not hear anything.

The (supersonic) aircraft produces sound throughout its flight (i.e. while flying towards as well as away from you). 

The sonic boom you hear is the sound produced by the aircraft when it was coming towards you.

For how this works, see this question and the Wikipedia page.