Always lip-synched. When they sing on parade floats, it is live singing. Why not the Broadway performances?
This very thread was started last week. Ah well, its posted again.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle