The MMSYSTEM module of Windows provides the interface between client applications and audio device drivers-applications do not call the drivers directly. When a client application calls a low-level audio function, MMSYSTEM translates the call into a message and sends the message to the appropriate device driver. When a device driver must notify a client, it calls MMSYSTEM with a message, and MMSYSTEM routes the message back to the client application, as shown in the following illustration: