Go to the participants list and click Mute All. For a standard Webex meeting or Personal Room meeting, uncheck Allow attendees to unmute themselves to prevent attendees from unmuting themselves until you grant them permission to, and then click Mute All.
https://help.webex.com/en-us/nz09ab5/Webex-Mute-or-Unmute-Other-People-in-a-Meeting