Interesting question - what I'd recommend is practicing once or twice with mouthing the words but not actually speaking. You can definitely do that and often it's that process of transferring the words from the screen to your lips that's the real value-add, and not actually hearing yourself say it. Much like writing can help you to do something with what you hear, and therefore better remember it, actually making the movement of verbalizing them is what helps you to better cognitively process them.
I wouldn't assume that you can just speak - it's probably going to be the proctor's call as to whether you're distracting others, and why invite some kind of penalty or unwanted distraction if you don't have to?
Here's the official word from GMAC:
Disruptive behavior in any form will not
be tolerated. The test administrator has sole
discretion in determining what constitutes
disruptive behavior.
I'd plan on being able to mouth the words silently, but anything above that at least invites the proctor to get involved or another examinee to ask for involvement, and I'd imagine that the time it takes to be "scolded", and the anxiety that it creates, would be more detrimental to your test than the practice of making audible noise would be helpful.