The only bit in Phantom that is pre recorded is the title song. They go back to actually singing when the phantom starts his line " I have brought you..."
I took a tour backstage of Phantom. There is a little montor on a shelf that is literally in the corner of the backstage area. There is a headset peace and little mics that are used for the actress and actor playing Raoul and Christine. That is where they sing there bit in the end of the show off stage and there bit in the reprsie of all I ask of you. It sounds as if it is pre recorded but it isn't.
I think that the bits in Mamma Mia that are off stage singing are actually being done off stage with mics set up.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll