Nyx was just counting in her head before she was moving some, and soon enough launched into the song that they had written. What they had written for the climax of the movie and to others the song would seem difficult - containing a lot of high notes. But that is what they had decided to use to really hit the nail on the head, to show that this was the climax of the film, the important part that needed to be noticed.
"How does her voice not crack hitting those high notes?" one of the actress was questioning another in a whisper, just more shocked than anything else.