When a sound source is moving away from an observer, the observer will hear a lower pitch. This is due to<em><u> decrease in frequency</u></em>.
Pitch of sound depends on the frequency of sound waves and also on the loudness and spectrum of sound. A high pitch sound corresponds to a higher frequency sound wave and a low pitch sound corresponds to a low frequency sound wave.
<em><u>Based on Doppler's effect, which is the change in the observed frequency of a wave such as sound wave when the source or the observer moves relative to the medium of transmission</u></em>, <em><u>as the sound source is moving away from the observer, the sound reaching the observer will have a larger wavelength but lower frequency. Thus the sound heard by the observer will have lower pitch. </u></em>