I said "sur le balcon" to my native French-speaking grandmother and she said "au balcon" is correct, whereas "sur le balcon" is not. Just FYI.
no, "au balcon" is exclusively used to mean you are in the upper part of a theater.
Oh, that's interesting. Au balcon appears elsewhere in this lesson and the assumption in the comments beneath that particular entry is that au balcon and sur le balcon are two ways of saying the same thing. Thanks for clearing that up.