If you turn the volume knob, it will adjust WHATEVER is currently being outputted at the moment you turn the knob.
FOR EXAMPLE: Lets say you are operating the FM radio, and Navigation together... When the radio is playing turning the volume knob adjusts the radio, when the NAVIGATION voice is speaking, turning the volume knob adjusts the NAVIGATION voice.
You will also notice that when you turn the volume knob, there is an icon inside the volume graphic onscreen that illustrates what output you are adjusting. A music note icon for music, a map icon for navigation, a cell phone icon for phone volume, etc.