Question

Explain how a driver of a moving car hears a siren but
can NOT hear the pitch of the siren changing.

Answer 1

The phenomenon has to do withthe Doppler effect or shift. The driver, i.e. receiver of the acoustic signal, is moving at a certain non-zero speed, so the perceived acoustic wave frequency (which determines the pitch) is different from that emitted by the source (i.e. the siren).