Registers a callback to be called to allow the game to access metering data from any mixing bus. You may use this to monitor loudness at any point of the mixing hierarchy 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 bus ID. Call with in_pfnCallback = NULL to unregister.