This is the Media Volume control. It allows you to control the volume of the PC, a laptop, or the Blu Ray Player (NOT the microphones).
Pressing the Up Arrow raises the volume in the room and the Down Arrow lowers the volume in the room.
Press the Mute button to silence the audio from all AV devices (except microphones). To restore the volume, just press the Mute button again.
When the Mute button is red, that indicates that it is active.