It is beyond rare for movie musicals to use live singing. It's not because movie actors can't sing but because movies are shot with so many takes that it can be physically impossible for a performer to keep their voice going for the hours and hours required. There are a couple of examples of live singing in movie musicals - I understand that Bette Midler did live singing in the TV movie of Gypsy - but there's usually simply no good reason to do it.
But according to the UK's (not usually that reliable) Sun, director Tom Hooper will be asking his actors to do all their singing live in Les Miserables, the adaptation of the hit musical. "First they have to learn the complex songs, then they'll have to get it right on set in front of the other stars and crew," an anonymous source tells the Sun.
I think this means one of two things: either Hooper is shooting the film as a play, setting it on a stage, or the source is mistaken (that's not counting thing three, the source is full of shit). If Hooper shoots the movie as a show he can do the whole thing live performance style; I think this would make a pretty shitty movie, frankly. The other option is that since Les Mis is pretty much an opera - all dialogue is sung - Hooper could be recording the interstitial stuff live, but doing prerecordings for the big numbers. This would make the singing dialogue come across as more natural.
Any news about this film is of interest to me, so I'll be keeping an ear to the ground.