Hannah was really grossed out at cutting open Cliff, who definitely was not singing while she opened him up. She licked her lips before she started practicing the spell. Her father is an obliviator and she knew she was going to ask him if Professor Carrow was actually telling the truth.
She didn't do very well at first, but the more she practiced the better she got. By the end of the lesson her brain was almost full jelly and Cliff wasn't singing anymore.