Carter is convinced she sounded better in the 2007 musical Sweeney Todd: The Demon Barber of Fleet Street because technicians boosted her vocals.
"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," dailymail.co.uk quoted Carter as saying.
"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 is quite daunting because you have got 50 people watching you, filming you and you have just got this thing in your ear with the tune and you're the only one singing. And it just feels weird," she said.
Carter, 46, 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.