The Warden has the rights to request players to stay off the microphone to help himself giving clear orders, you can kill Red players talking through your orders if you give a warning first.
But you cannot restrict players to be muted for the entire round, you can request 30 seconds or the the entire start of the round to speak for yourself as Warden.
You as Warden can't restrict players from using the chat or force players to type in the chat. (Jeopardy/Kitchen Minigame is the only exception)
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A great idea would be to be to have them unmuted after 30 seconds and then do what yoyo said and request them to stay off mic. Or find a way to keep the dead reds muted, but the likeliness of that happening is a little slim. So I would like to see the time shortened, (like glitches causing reds to be unmuted early in the round or once when they got unmuted when the round started.