I have been told 'everybody' is singular. However, there was a film named "Everybody Sing".
What are the differences between "everybody sing" and "everybody sings"?
Which is correct? the former, the latter, or the both?
Which is more natural?
They mean different things.
"Everybody sing." by itself would be a command telling everyone to sing.
"Everybody sings." is a statement that everyone sings.
I don't know the film in question but here is how I would differentiate between them.
You would use sings in standard sentences of subject + verb:
Everybody sing is a command or instruction eg if you are leading a choir.
Everybody is to get everyone's attention, the command is Sing