IToiAudioOutputService is used for controlling audio outputs. It allows the client to control how to decode and mix audio content and route the modified/unmodified content to the outputs.
The sound volume is controlled by adjusting the level and if the output should be muted or not. For instance, adjusting the sound level when the output is muted will not make any audible difference. However when unmuting, the sound level will be restored to the new level.
See also: TOI Audio Output Service Interface