The frequency of a waveform will seem to vary according to the relative motion of the waveform source and reciever.
This phenomenon is known as the doppler effect.
Light will seem to become bluer when the source and reciever are moving towards each other and redder if they are moving apart.
With sound, the pitch of the sound varies, becoming higher when source and listener are moving towards one another and becoming lower when they are moving away from one another.
Doppler effects are applied to a sound by calculating the relative velocity of the 3D sound buffer and listener, along with the current doppler factor, which acts as a multiplier to the calculated doppler adjustment.
The default doppler factor of one approximates the real world behavior of sound.