Give her a taste of her own medicine. Some night when she's quiet, gab to your hearts desire on the phone with someone and in an usually loud voice. Perhaps she'll get the clue.
Seriously though. I have this friend who comes from a large family and she has this voice that CARRIES everywhere. It probably didn't help that she worked in a bar for a while and was used to yelling over people and talking loud to be heard. So now that I got her a part time job at the jewelry store where I work I've reminded her where she works because talking LOUD like that just scares people off!
Something to think about!