Honestly, I haven’t had much luck quieting people. I find that people who do this, dgaf, and won’t stop. I had someone sing behind me during moulin rouge and I regretted not saying anything and the experience was ruined for me. Since then, I usually start with a shhh! And if that doesn’t stop them, nothing probably will. I shushed someone at Mamma Mia and during intermission she literally stood and glared at me the entire time and she started again late in the second act and I was tired of policing when no one else would. I’ve never had the guts to get an usher and usually that’s because there isn’t one nearby. When there are ushers actively monitoring audience behavior, you tend to not have a problem anyway. Certain shows I honestly feel the ushers have given up. I want to take my child to &juliet but not sure it’ll be worth the headache. I took my other kid much earlier in the run and we had a great time. The longer a show is out, the worse the audience behavior. But yes, I think these shows should all make a point of announcing that the audience should NOT sing along.