An ACM driver can provide a custom About box. This About box is displayed by the Control Panel’s Multimedia applet. If the driver does not provide a custom About box, the Multimedia applet uses a default About box.
When the driver receives an ACMDM_DRIVER_ABOUT message, it should call DialogBox (described in the Win32 SDK) to create and display its custom dialog box. If the driver does not provide a custom dialog box, it should return MMMSYSERR_NOTSUPPORTED when it receives an ACMDM_DRIVER_ABOUT message.
To lessen the task involved in writing an ACM driver, and provide user interface consistency, it is better to use the default About box than to provide a custom About box.