Sent to a waveform output device driver to request the number of waveform output devices available.
Drivers can be structured in several ways. The simplest and most common drivers use one physical device to support one logical device. Drivers can also support multiple logical devices using a single physical device. For example, it can use a single DSP to support playback of both PCM and ADPCM data. Such a driver would return a value of 2 for this message.