The sugar, butter, flour line, the one that is heard right before Jenna starts to sing What’s Inside, is Bareilles’s voice. They also used, as their pre show no cell phone announcement, the song that Sara wrote. I know that if there’s a pre recorded track being used in the show, it has to be of the particular performer. For example, in Phantom, if the alternate Christine is on, they have to use that respective actresses voice recording. Same for the leads.
What I cant figure out is, how all that works as far as credits are concerned.