Opera house with a retractable roof, and the sky kept changing

I'm on stage and the roof of the opera house slides open like a camera shutter. The audience is still there but they're all looking up, and the sky keeps changing — first it's a clear night, then it's dawn, then it's full of floating doors. I'm singing something I've never learned but I somehow know every word. When I try to see who's conducting, it's just my own shadow, standing on the podium, turning the pages of an empty score.