The WIM_OPEN callback message notifies a client that a user-mode driver has finished processing a WIDM_OPEN message.
A user-mode waveform output driver sends a WIM_OPEN message to its client, by means of a callback, when the driver finishes processing a WIDM_OPEN message. The driver sends the message to the client by calling DriverCallback, passing the specified parameters.
The driver sends the WIM_OPEN message only if the client has specified a notification target with the WIDM_OPEN message.
Win32 SDK documentation states that clients receive an MM_WIM_OPEN message if the notification target is a window handle. WIM_OPEN and MM_WIM_OPEN are equivalent.
For additional information, see Notifying Clients from Audio Drivers and Transferring Waveform Input Data.