The eccentric Brit underwent intense vocal lessons to prepare for her role as pick-pocketing Madame Thenardier, and sang live in director Tom Hooper's big-screen adaptation of the beloved Broadway show.
But the actress was let down by her turn in Les Miserables and is convinced she sounded better in the 2007 musical Sweeney Todd: The Demon Barber of Fleet Street because technicians boosted her vocals.
She tells Britain's Daily Mail, ""I had lots of vocal training for Les Mis but I didn't improve as much as I thought I would. I watched it and thought... I was going to be so much better, I thought my voice was going to be so much bigger.
""I practiced and practiced and practiced but the thing with Les Mis is that it was real so that's what we sounded like. In Sweeney I had people making me sound better than I am but not for this.
""It's quite daunting because you've got 50 people watching you, filming you and you've just got this thing in your ear with the tune and you're the only one singing. And it just feels weird.""
Bonham Carter also ruled out ever starring in a live musical, adding, ""I would do another singing (film) role if someone else was foolish enough to employ me but I wouldn't do a stage musical. I don't have a strong enough voice. Maybe for one night but with singing it's like any muscle, it doesn't last.""