A lot of people think they can sing, and want to sing, but only can if they have a studio and auto-tune.  Live performances don't lend themselves to that device, which is why so many are lip-sync'd.  That might be where everything falls apart.  IIRC, the Les Mis movie used a lot of of the cast from the Broadway play, which means people who can really sing!  They're used to belting out to the back row.