I work in a high school, so normally we get a few people singing along, and the actors always complain..and I can understand why. It is so annoying!
OK, gather round, story time!
Last summer, The theater was performing a Mainstage (not a childrens theater) show, Seussical the Musical, and the Audience was being PERFECT, except for this one woman, who would sing VERY loudly to every song, I called on headset and asked an usher to please ask her nicely to stop....that didnt work. We had a little child playing Jojo, and after the woman ruined "Alone in the Universe" He came back almost in tears (opening night, young kid, first show ever...poor guy). So anyway, durning Intermission, I Changed out of my Blacks and into something Dressy, and courteously escorted the woman from he theater...she was not given a refund.(Included in the sectio0n about distractions IE.Cell phones, pagers, flash photography, there is a mention of Loud talking/singing, so thats how we got away with it

)
It Really pissed me off, because this one woman ruinded the first act for like the entire audience...
long story short..If you want to sing along...stay away freom my theater