How do you get a loud, bossy co-worker to quiet down and mellow out?

In our office we have a woman who insists on talking loudly at all time, and in an authoritative voice. She has a young child and talks to everyone in our office in a tone of voice as if she's talking to a child. This drives all of us nuts and we don't dare say anything or she gets pissy. She always has to have the last word in any conversation and escalates her volume the longer she talks. We can't even hear ourselves think most of the time, and fee like nervous wrecks at the end of the day. It really takes a lot of energy to be around this person 8 hrs. a day, 5 days a week!!!