My mom chatters incessantly and I'm going to ask her to tone it down.
Is her chatter constant?
She won't shut up and the plan is to let her know how annoying her voice is.
Sometimes people talk a lot and it is okay to get tired of them going on and on, but you shouldn't tell them to shut up. It will hurt their feelings.
It's probably going to hurt her feelings, yes. And though she is my mom, I want to make sure that she knows what she is doing.
Telling someone else to shut up, especially someone like your mother, is rather rude. Have you thought about in moments of her annoying your with chatter you could kindly let her know that you just want to be quiet and not talk for a bit. It's a request that can be made without being rude and she will probably understand.