Secondary bus hierarchies are structures of Audio Busses used to mix the content that will go in any other outputs than the main (TV or speaker) output. You can create as many secondary Audio Busses as needed for the various types of outputs there will be in your project. Examples of secondary outputs are game controller speakers, chat headphones, or the DVR-bypass output. Like your primary output structure, the secondary structures can have any number of child busses and Auxiliary Busses.
Sounds routed to the master secondary bus hierarchy will be sent to the secondary output using one of the two following approaches:
Setting the Output Bus property of a sound directly to any bus in a secondary bus hierarchy. This is the preferred method for sounds that are normally tied to only one secondary output instance. For example, player-initiated gunshots, tennis racket whacks, PDA sounds, gameplay feedback, and so on.
Routing a sound through any bus in a master Audio Bus hierarchy and adding a user or game send to an Auxiliary Bus inside the secondary bus hierarchy. This is the preferred method if the same sound is going to be heard in multiple outputs and the TV at the same time, such as with a spy camera or announcements.
The bus hierarchy only defines the mixing structure. To associate that mix to a specific output, choose the appropriate Audio Device ShareSet on the corresponding master bus. See the list of Built-in Audio Devices provided with Wwise.
In the case of player-related outputs (such as game controllers and headphones) which could have multiple instances in the game, it is important to know that the associated mixing hierarchy will be duplicated for each player. The project only defines the mixing "recipe" for a specific type of output. Actual routing into which copy of the structure will depend on the Listener and Game Object associations set up by the programmer.
By default, the authoring application will route all sounds to the main sound card so you can hear your sound design. However, you can select different hardware devices for testing by going into the Authoring Audio Preferences dialog from the Audio menu.