I would suggest a raise your hand icon. This icon also noieifts the moderator (unobtrusively-maybe a flashing icon or something). Once a hand is raised the moderator can then click to confirm and allow that person to talk. In addition, the system will only allow one person to be active at once. Another person could raise their hand and wait for the moderator to give them access to speak. The moderator can take control away and speak themselves at any time and then re-grant speaking control at will.