Same things happens with group singing, but I'm working on a solution that seems to work pretty well. Instead of telling people to quiet down, I suggest that they listen for the blend of the sounds of the voices and instruments, rather than just listening for their own voice. That has made a lot of difference in three of the groups I sing with.
Now I'm working on two choirs to get them to let the sounds of the voices blend before they reach the microphone, rather than having people fight to see who gets closest to the mike. That part of the experiment isn't working very well yet. I've sometimes resorted to putting unplugged microphones in front of people, just to get them to stand where I want them to stand.
-Joe Offer-