Your concern, while valid, I believe can be mitigated. Not entirely, mind you, otherwise it wouldn't be much of a disability, but enough to help. As I mentioned, she can read lips and understand what's being said by that alone. So if she can see the speaker she can understand what is being communicated.
Moreover, it's not that she [i]can't[/i] talk; she chooses not to, primarily resulting from a lack of confidence.

She could have a hearing aid, but from personal experience, it may not help that much. While she could be alert to sounds in general, distinguishing one sound from another may be difficult.

I wasn't really wanting her magical girl power to be hearing-related anyway. From I was reading, it's more an expression of the character's personality and mannerisms.