Every 3D sound buffer and 3D listener has both a position and a velocity. From a graphics and animation standpoint, these characteristics seem quite similar. As one would expect, the position of a 3D sound buffer or listener represents its location in 3D space. However, the velocity of that buffer or listener does not represent how fast the object is moving in space. Rather, DirectSound uses the velocity of a given buffer or listener to calculate Doppler-shift effects.
Velocity adjustments can be useful if you would like to exaggerate the Doppler shift of an object. For example, imagine that you want the sound of an oncoming race car to seem as though it is screaming past the listener. If you exaggerate the effects of Doppler shift for the listener, the exaggeration will affect all sound buffers that the listener hears. To exaggerate the effect for the race car alone, you could increase the velocity setting for the car's 3D sound buffer.
The system handles cumulative Doppler-shift effects for you. If your application's listener and the sound source have velocities, the system automatically calculates the relationship between the velocities and adjusts the Doppler effect accordingly.