An application can set and retrieve a sound source's position in 3-D space by using the IDirectSound3DBuffer::SetPosition and IDirectSound3DBuffer::GetPosition methods. A position is expressed as a vector, relative to either world space or the listener, depending on the processing mode.
To set or retrieve the velocity value that DirectSound uses to calculate Doppler-shift effects for a listener, you use the IDirectSound3DBuffer::SetVelocity and IDirectSound3DBuffer::GetVelocity methods. Velocity is measured in distance units per second—by default, meters per second.
Note that velocity is completely independent of the actual position or movement of a buffer. It is entirely up to the application to set the appropriate velocity for a buffer.