The other answers seem to focus on personal experiences related to yours, I'll instead focus on my experiences from the point of view of your co-worker. I mumble/gesture/talk to myself often. It's who I am and it's how I work.
The first thing that you should realize is that she may be doing it unconsciously. Asking her to stop likely won't result in anything. Instead, she'll be silent for a day or two, and then regress back to the same behavior. Bringing it up further will make her conscious of her behavior, and will detract heavily from her job.
The second thing that you should understand is that this is the way some people work. The benefit to talking out loud to yourself is the fact that you use a 'different' part of your brain to analyze what you're doing. I (and I suspect your co-worker) am an auditory and spatial learner. What that means is that if I listen to something, or I somehow use my spatial recognition, I understand it much better than purely visual/textual. Thus, whenever working on a complex problem, I talk and use gestures to activate my auditory and spatial analysis.
Now, obviously if she's talking at a loud volume, I don't see the harm in bringing it up. But quietly murmuring I believe is a lot to bring up. If you do, I sincerely believe you'll be impeding her ability to work.
Some of the other answers have suggested playing music / other workarounds for this situation, and have been asked to provide a direct answer rather than challenging the frame of the question. My issue with this is the fact that as someone who understands your co-worker's idiosyncrasies, I only see confrontation ending badly.