Sent to an audio mixer device driver to request the number of mixer devices available through the mixer driver. This message is normally sent once after the driver has been booted and enabled by the mixer manager. The number of available devices returned for the MXDM_GETNUMDEVS message is added to the total number of mixer devices available in the system. It is this total value that is returned to an application that calls mixerGetNumDevs.
Audio mixer drivers can be structured in several ways. The simplest and most common drivers will use one physical device to support one logical device. Drivers can also support multiple logical devices using a single physical device. For example, some audio cards allow more than one card to be installed in a single machine. If the device driver can simultaneously support more than one card, the driver should return the number of physical cards that are currently available.
The value returned by a mixer driver for the number of devices it supports cannot exceed MAXMIXERDRIVERS. Also note that the total number of mixer devices returned by all mixer drivers cannot exceed this same value.