Reading out loud at the test center

While a low mumble will probably not be noticed by anyone, there is a chance that people will complain to the proctor. In that case, you will probably be warned or maybe even ejected. The latter situation is unlikely, but we cannot rule it out. As long as your mumbles stay low, then you should be okay. Still, try to practice without doing that just to be safe. Be prepared for anything and try to stay flexible during the test.

yeah i mumble too. when i got the 550 they didn't say anything and it was a full house. I figured if im paying this kids $250 i ought to be able to hoot & holler & raise hell if i wanted to

I don't think that the fact that you pay $250 is reason for anyone to sing or shout or whatever. Not sure why, but the attitude is kinda not that great. If you really think about it, for just $250, they're letting you get one component of your B-School Application. It's all relative.

But about your question - I think that it's okay as long as you're not disturbing someone else.

Everything is relative, really. On one side, they indeed are letting me get a portion of my application for $250. On the other hand, they make the exam pretty demanding which consumes a lot of time, leaving little time for the rest of application. So, there are few ways to look at GMAT... It is necessary nonetheless.

A very interesting question OP. I myself would prefer common courtesy to make sure I do not disturb anyone. That being said, this test is very important, so to each his own.

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.