Currently there is a notification sound that plays when Screen Sharing is started. This sound by default will play out of the speakers of all Presenters and Attendees to indicate when screen sharing has started. It can be disabled individually if you would prefer to not hear it. If you change this setting it will be remembered next time you log in, as long as it is the same computer.
In the admin page Under Modify Room Settings choose the appropriate room. Then, under Room Function, Audio Notifications un-check the Projection Start check box. Then click the Update Changes button below. You will have to restart the room in order for the changes to take effect.