Frequency changing element in an FM transmitter
The mic outputs some audio ac voltage which is as an addition on the supplied dc between its terminals. That ac varies directly the operating point of the transistor. The internal junction capasitances of the transistor vary and that affects directly to the oscillating frequency. 

Addendum: For example think the capasitance between c and b. Its changes affect through other capasitors on what is in parallel with the coil.

The fm deviation is difficult to pretend. For this reason fm is often realized by having a varactor in the resonant circuit an applying the audio ac to it.