If the sound icon is not highlighted in the tool bar the sound function is not activated on the monitor.
To activate/deactivate the sound, press the sound icon in the MENU:
if the icon is highlighted the Sound Function is activated
if the icon is dim/not highlighted the Sound Function is deactivated
Press OK to save this setting. If you do not press the OK icon after approx. 30 seconds the setting reverts to the original and you will need to begin again