Registers a callback to be called to allow the game to access metering data from any output device. You may use this to monitor loudness as sound leaves the Wwise sound engine by accessing the peak, RMS, True Peak and K-weighted power (according to loudness standard ITU BS.1770). See Advanced Mix Customization Using the Speaker Matrix Callback for an example. The callback must be registered once per device shareset ID. Call with in_pfnCallback = NULL to unregister.