This message is used in an MMDRV_MESSAGE_PARAMS structure passed to the WAV_IOControl function to request a waveform output driver to return the number of device instances that it supports.
The driver returns the number of waveform output device instances it supports. For Windows CE, the default MDD returns 1.
The ACM sends the WODM_GETNUMDEVS message by calling the audio driver’s WAV_IOControl entry point through the DeviceIoControl function.