I have a minor dislike of those people who go sssshhhh all the way through the first two or so verses of an unaccompanied song in an effort to make everyone else shut up and listen. If they wanted to hear it, why make such an irritating sound which often also embarrasses the singer. The rest of the people will either quieten down when they hear whats going on, or not. If you must do something, eye contact is less obtrusive and more effective.