they lip-sync at the thanksgiving day parade (macy's hosts it but it is for the holiday) because its cold, and outside. and due to the wind issue they'd have to put a rather obvious windscreen on each mic. (imagine elphaba running around with a grren foam ball on her forhead). They had the kid from the boy from oz sing live that year with a rather large lapel mic and it was distracting. And of course as i'm typing this margo already answered the question but i'm posting it anyway since it expands on the answer
On Sirius radio they were saying that several shows have "backing tracks" to make the big chorus numbers sound fuller. They included Hairspray as one of these shows. I am glad Marc cleared that up because I was considering not seeing it because of this.
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