Castiel wasn't a particularly talkative person normally, not even when he was outside of a vessel, but speaking was pretty much his own method of communicating; he hadn't been created human, so he had never had reason to truly learn how to speak without words. Gestures, body language, facial expressions; he was still getting used to the very basics of them, and couldn't use them to express himself in the way that most could.

So he felt most comfortable with writing, which was how he answered her again.

My voice is gone. I don't know why.