As a moderator, you can restrict all participants to activate their microphones in the session.

Go to Settings in the navigation bar.

A popup window will appear showing three categories, go to PARTICIPANTS > Can share microphone.

Turn it on to allow your participants activate their microphones.

Turn it off to prevent participants activate their microphones.

This will only affect participants.
Other moderators in the session can still turn on their microphones.
These settings have no effect in Rooms since all users become moderators there.