defenses said: "Wow! This is a very old thread but it’s fascinating to see that some things don’t change. The title track you hear is recorded until “sing my angel of music” then again for the high note. I’m pretty sure the actors usually sing over it (at least it sounded like they did when I was close), and sometimes you can clearly hear the Christine hit that note at the end over the recording.
(They stopped using the doubles, though, so the Phantom and Christine you would see were the “real” ones.)"
Just for clarity, Christine doesn't sing at all during the song. (Not that some haven't sung anyways, but the whole song is recorded for her.)
And for the poster that said "I have also heard that it's up to the performer playing Christine" - that's also not how they do it. It's the same thing night after night for consistency, so it's always recorded unless the tracks go down, which is VERY rare, especially once theatrical sound technology got further along. I'm not even sure that happened once the show got into the 2010's and they upgraded their sound system.