There is some interesting research that suggests an explanation of why many players, such as Sharapova, vocalize at the moment they strike the ball. A close look at the motor cortex reveals some clues. See the figure below for how the motor cortex is laid out in the human brain.

	"There is also the suggestive observation that the representation of body parts on the pre-central gyrus (motor strip), which is where conscious motor activity is activated, runs from arm to hand to face to mouth to larynx as one gets closer to the sylvian fissure (Calvin 1983a). Clearly, there is some close connection between speech and throwing which may reflect a common underlying mechanism [Freeman-Langer mesoscopic assemblies]. Calvin suggests that, in effect, linguistic ability was able to develop because of an increasingly accurate sequencer in the dominant hemisphere that was initially utilized for intermittent throwing." Brackets added by me.
Motor Cortex and the Homunculus	Excerpt from Dissertation of Paul Thomas Schoenemann, U.C. Berkeley, Anthropology	Click on image to see video

Inhibiting this vocalization may even inhibit the associated ballistic motions that utilize the hand (and the Type IIb muscle fibers involved). More to the point, consciously inhibiting this vocalization as a result of respecting social norms may result in an increase in the rigidity of the stroke which is detrimental to execution, especially when nervous. It is advisable to experiment with vocalization in practice to see what relaxing effects arise, resulting in a more accurate control of the racquet path.

To take this argument further and combine it with other research, we note that predators vocalize at the point of attack and this may be related to the close connection between the motor pathways for ballistic actions of the predator and the seat of vocalization. As vocalization has an paralyzing effect on the prey, vocalization at the point of attack would be favored as a successful survival attribute through the process of natural selection.

In summary, vocalization is an automatic response that occurs in conjunction with a ballistic action using the hand, and likely the body as well. Further, since vocalization at the point of attack has a paralyzing effect on the prey, it was favored through the process of natural selection. Inhibiting this process can lead to a rigid stroke production that may even spawn unforced errors when nervous. Alternatively, vocalization, due to how we naturally process tone, has a paralyzing effect on the opponent while making the attacker more aggressive.