So in your campaign she's the mumbling hag. It doesn't really matter, as long as you had fun. In my opinion you can get more fun out of 'The Harrowing' by roleplaying as many encounters as possible, than by fighting every harrowed creature. So more power to you!
To answer the question of how she could have bargained without talking: Options for bargaining that don't include talking would be to use a form of sign-language, or to write, or even draw.
An option to have her "talk" properly would be to grant her ghost sound as an "at will" spell-like ability.
I recently ran that encounter and handwaved it that she had the ability to speak regardless (something like Are's 'ghost sound' suggestion). I made the executive decision that just as many of the NPCs in the Harrowing aren't precisely what they're represented as in the text (the Nightpeddler not being an actual Denizen of Leng, for instance), I could fudge a bit with Sonnorae.
If nothing else, you could say it works like the illusion power the folks in the playhouse have -- it's just a quirk of the world that it works the way it does.
I have an undead skeletal assassin in a game I play, which cannot talk. When it seeks to parlay, it has the other party meet it at some unhallowed location. And I have it hunched over a piece of paper, pricking its clawed skeletal finger into a vat of ink and scrawling on the page.
Only when the parlay is over and the assassin leaves does the other party realize it was pricking its claw into a living creature. Usually a dove, though if the other party represented a church with a sacred animal, it would probably be that instead.