She clearly said she was fed up. She wasn't confused, or unsure of what to do. She just didn't want to. Two choices, with one being the obvious winner. How hard could it be?

I also know that singing with emotion is difficult if you don't carry that emotion, but whether that will have an effect or not was unknown to Ranka and everyone in that scene, so it is a non-factor. As far as everyone knew, she just had to open her mouth and spout some Ai-mo stuff to have an effect on the vajira.

@Kraco - you are looking at it in retrospect. It doesn't matter what actually happened, since we are talking about Ranka's choice based from what she knows at that time.