
DeathlessOne |

How did she lose her ability to speak? Was it due to magical tampering or through lack of use? A lot of how we speak is tied to muscle memory, so if that is how she lost it, it would just be a matter of relearning the movements and breathing techniques requires to produce language.
I'd probably have her learn how to control the sounds she can already make, slowly build it into a sort of controlled hum or vocalization of vowel sounds and grunts, and then slowly build back up to fine motor control required to articulate clearly. Definitely will need to have her study with one of her party members or someone else during downtime so that they can demonstrate the correct ways of vocalizing the words she can read.

DeathlessOne |

Interesting. Not sure how that would work given standard cosmology but if we assume Wish level magic, I've got no problems with suspending disbelief.
Something else you might like to know: People that have issues with stuttering or other speech impediments find that activating a different portion of the brain in an effort to communicate helps bypass those limitations. Singing, for instance, can help alleviate stuttering for some people. Perhaps you can have her learn to vocalize through singing and then it would translate into a more sing-song type of speech?