Went to an open-air production of HMS Pinafore last night and the woman sitting behind me would sing along with the actors every so often. It wasnt all the time - just a few lines occasionally. I found it really off putting. I turned round and gave her a
look a few times but she continued.
Eventually I turned around and politely said: "can you please stop singing".
Her facial reaction was a bit but she did shut up after that.
I think its ok to sing if it's a singalong production by the way - e.g. Rocky horror or special screenings of Sound of Music or Mamma Mia.