Question

If the pitch of a sound decreases and the sound source is moving, is the sound source moving toward or away from the observer?

Answer 1

The sound source is moving away from the observer.

Pitch is frequency. Frequency is inversely proportional to wavelength.

As the source moves away, the wavelength of the sound will be increased as measured by the observer. This means the frequency will be lower and, hence, the pitch.

Answer 2

Look up Doppler Effect