If it's allowed, then I am extremely verbal during all forms of sex, though as I approach orgasm I temporarily lose the ability to form coherent strings of words and mostly just whimper until climax is achieved, at which point I roar in a rather bestial fashion.
If there is a need to remain quiet, I'm capable of that, also. During sex I'll mostly keep my eyes closed since it's visual stimulation that loosens my tongue, in general. If I happen to be overcome by the need to say something, I'll lean in close and whisper gruffly in my partner's ear. As orgasm approaches, however, my self control is remarkably diminished. Thus, I find it necessary to take some drastic measures to stay silent. An obvious step is to bite something, be it my tongue, lip, fingers, hand, or arm. However, it's important to remember that biting does not truly muffle any noise that might escape your throat - what it's supposed to do is cause you enough pain to bring you to your senses. Gripping a fistful of your hair by the roots and pulling sharply is helpful. One thing I like to do is cross my arms over my chest, dig my fingernails into my shoulders, and scratch an "X" into my torso. This in conjunction with biting my lip has been my preferred method of shutting the fuck up lately.
I definitely prefer a loud partner to a quiet one, but the important thing is that they enjoy themselves, really, and it is certainly possible to do so without making a lot of noise.
For some reason, thinking about this really turned me on.