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