The change from a high to a low pitch is a phenomenon known as the Doppler Effect or the Doppler shift. Sounds come in waves and are known as sound waves.
The Doppler Shift or effect is relevant to the location of the person who is hearing the sound. When a person is moving closer to the source of the sound, the wavelengths become shorter and they become low pitch; but when the person moves away from the source of the sound, the wavelengths become longer and become high-pitched.