Registers a callback to allow the game to modify or override the volume to be applied at the output of an audio bus. The callback must be registered once per bus ID. Call with in_pfnCallback = NULL to unregister.
Note: The bus in_busID needs to be a mixing bus.
Note: The callback function will not be called for the Master Audio Bus, since the output of this bus is not a bus, but is instead an Audio Device.