The AUDIO MUTE button toggles on and off - press for on and again for off and again for on et cetera.

This will mute the audio to the audio amplifier and will affect the PC, laptop and auxillary (if available) audio inputs. 

If you have no audio check that this button is not on.

The button panel controls the AV switcher in the desk rack.  The AUDIO MUTE can be selected using the AV switcher if the local button panel is not working.