The dev and the sound designer just need to agree on the name and range of the Game Parameters (for example "Music_Volume", "Voices_Volume", "Main_Volume", etc), and then the sound designer can add Volume curves to busses based on those Game Parameters, and tweak the curves as they see fit (even in real-time while Wwise is connected to the game, so you can tweak the volume controls in the game without having to modify the code, recompile, etc).
Note: Volume is "additive" in the hierarchy. This means that if a parent bus has RTPC on the volume, and one of its children also has RTPC on the volume, both RTPC curves will contribute to lowering the volume of sounds playing through the child bus. For example you could have RTPC on the volume of the Master bus for the main volume control, and RTPC on sub busses for music, voices, ambient sounds, etc.
Note: RTPCs on busses are not dependant on one particular game object. When setting RTPC values that are destined to busses, be sure to use AK_INVALID_GAME_OBJECT as the game object ID. Game object-specific RTPC values are ignored by busses.
