They all seemed believable to me, except the one I speak, which is by-God Southern.

Her Charleston accent, like that of so many awful Hollywood movies, sounds like Foghorn Leghorn at a Civil War reenactment.

Good Southern accents by non-Southerners:

Miranda Richardson in "The Apostle"
Jennifer Aniston in "The Good Girl"
Kelly Macdonald in "No Country for Old Men"

No sure why I picked all women there...