Sounds to me like she's engrossed in what she's saying, and is running the thought over again in her own head and verbalizing it while so doing.
What can you do about it? Danged if I know.
Anyone I've ever lived with had numerous little habits that started to drive me nuts after a while. My solution has been to live alone, but that's not most people's chosen solution.
I guess acceptance of people's oddities is a big part of learning humility and compassion...
Then again, you could fight back by repeating everything yourself...by repeating everything yourself. Maybe she'd notice, maybe she wouldn't. Maybe she'd notice, maybe she wouldn't. Maybe she wouldn't. If it started to bug her as much as it bugs you, if it started to bug her as much as it bugs you, then maybe she'd become more aware of the habit. Maybe she'd become more aware of the habit.
Or maybe she'd go for you with the frying pan. :-)