As a performer, I'm always put off when people start singing along uninvited. Or clapping!! If I'm playing a concert, I want to present the music in my own way. Some songs are better with audience participation and some aren't.

As an audience member, I REALLY don't want to hear anyone singing except the person I came to hear, again, unless the performer asks the audience to join in. I've always thought that uninvited singing is the height of insensitive egotism. If you want a participatory experience, go where that is offered, or make your own.

It may be worse at rock concerts than at folk concerts. I remember seeing Jethro Tull a few years back and trying to hear Ian Anderson, who has one of my favorite voices. I had to ask, multiple times, for the donkey seated behind me to stop braying along.