What Bert said "Just sing the songs the best you can, and get on with it."

You pay some respect to the language used, if it's not quite your language, but you basically sing in the way that seems natural to you.

And formal grammar doesn't come into it, any more than it does in conversation.

As for "ain't", that been good English since the 18th century, posh people's English just as much as common folk's.