The MM_ACM_OPEN callback message notifies a client that an asynchronous ACM driver has finished processing an ACMDM_STREAM_OPEN message.
An asynchronous ACM driver sends an MM_ACM_OPEN message to its client, by means of a callback, when the driver finishes processing an ACMDM_STREAM_OPEN message. The driver sends the message to the client by calling DriverCallback, passing the specified parameters.
For more information about the use of MM_ACM_OPEN, see Notifying Clients from ACM Drivers and Converting Data Streams.