When music joyfully calls out to us,
It takes a moment for us to perceive
The invitation to respond, and thus
We pause, not wishing to appear naïve.
But with the second call we understand
Just what we are expected to repeat.
It’s both request and cheerful reprimand
That silence makes the music incomplete.
To ask the audience to join the song
Turns passive listener to chorus member.
The phrases are not intricate or long,
And with each repetition, they remember.
So raise your arms and voices when you’re told-
Your pleasure will increase a hundredfold.
Libby Weber 
We could use another Mitch Miller and his sing along concerts and records. This ages the old man!
Pop
Some contemporary bands (the Indigo Girls and The Decemberists spring to mind) work the inevitable audience singing into their concerts, which is loads of fun. But yeah, actually being invited to sing along is an all-too-rare occurrence these days.
Yes! Singing along is WONDERFUL. I wish people did it more at operas and musicals 🙂
I’ve been to Dr. Horrible and Once More With Feeling sing-alongs, but it’s not the same, is it? One of these days, I’m going to attend a performance of Nabucco, and they’ll do an encore of Va Pensiero and encourage the audience to sing along. AND I WILL BE READY!