In the ghostly theatre, there is a balcony far too high and I can't reach..

If your talking about the room with the first man eating plant, or the second, then you must get the man eating plant underneath the balcony and beat it with the whip till it opens, then jump on it and double jump onto the balcony. You can't jump high enough to land on the balcony, but you can jump high enough to grab the balcony and pull yourself up.

If your talking about the one on the 1st floor in the room with the falling spikes I couldn't find a way up there. But there is another spike room on the 2nd floor which is a lot easier to get to. Once you reach the 2nd floor room with the falling spikes look for an area where the spikes wont fall over by the wall. If you see a pipe on the wall( you may have to jump to see it} double jump and whip onto it and you will be able to get the artifact. I'm not a 100% sure what it is but I believe it's Lucifers Sword. Hope this helped out.