You shoot movie musicals with recorded music. This is how everybody does it, simply because recording live singing is extraordinarily difficult and completely, totally imperfect. Almost nobody ever shoots musicals with live singing; the last I can think of is Bogdanovich's At Long Last Love, which has Burt Reynolds crooning. The film is better than its reputation, although some of the singing is... less.
Anyway, Tom Hooper is following in Bogdanovich's footsteps and is shooting his adaptation of Les Miserables with live singing. And this is a show that requires real, serious chops. This new featurette gives a behind the scenes look at how they're making that happen - I really recommend checking it out to see what a remarkable display of balls this film is.