I'm always amazed how people at a microphone don't know how to quiet the room.
What works best: Saying SHHHHHHHH (as if hushing a baby or quieting someone speaking too loud) into a microphone. Next best: hitting a knife against a glass to make a sound (like for a wedding toast). A gavel could also work but if the speaker is not chairing a meeting that sounds too harsh.
What never works (or takes forever to work): Saying over and over "Can I have everyone's attention, can I have everyone's attention" or "would everyone please take your seats, we're about to get started." Staring at the crowd waiting for them to naturally quiet doesn't work either.
Remember, to quiet a room, just say make the SHHHHH sound into the microphone. It works quickly.