Anne Hathaway can belt out a tune. I mean, she did a hell of a good job. Russell Crow used to be the lead singer for a rock and roll band (before he got famous) and he does an admirable job. Hugh Jackman can't sing his way out of a paper bag. His overwhelming vibrato distresses me on a cellular level. It's like he doesn't know what note to hit, so he hits a bunch and figures that is close enough.
The little girl playing the young Coset is precious, and she will inevitably turn into a narcissistic Hollywood type. But, for now, she's just a cute little girl who can hit a note. Helena Bonham Carter in a non-Tim Burton movie? How'd she get out of that exclusive contract? However, when I think of that role of a scraggly, slightly insane woman, I think of her right off the bat. Sasha Baron Cohen as the comic relief was... okay. He's no singer. And he was trying so hard to be funny that it was hard to follow what he was saying for a good portion of his singing.
I don't recognize the guy who played Marius, but are we supposed to believe he's straight? And he sings like he has a cue ball stuck in the back of his throat. He had a pretty good falsetto, though - I'll give him that. The older Cosette is played by the chick who was the daughter in Mamma Mia. Didn't like her much then, and didn't like her this time, either. She was still better than Hugh. Man, he sucked. Oh, and more than an honorable mention should go to the woman who played Éponine. She sings well - way better than Cosette.