If the camera has two-way audio configured, the menu item becomes active. Two-way audio can be in three different modes.
In the closed mode, the audio channels are not open. In the listening mode, the user will hear audio from the camera and any potential audio going to the camera from any other VMS client.
The audio channel from the current user is not open. In the talk mode, the user can hear the audio from the camera, and the audio channel from the current user is open, so the audio is broadcasted to the camera. The toolbar icon has red (to camera) and green (from camera) volume indicators, which show if there is audio going to or coming from the camera. The channel volumes can be adjusted from the sliders.