THE BODY'S in great shape, the clothes are right out of "Dress for Success" and the tan is California-surf-bum-perfect. You're impressed.

And then (he, she) opens (his, her) mouth . . .

The right voice, more and more people are realizing, is crucial.

In their desire, however, to come across as sultry, sexy, macho or authoritative, an alarming number of people are misusing their voices. "Voice suicide," claims Los Angeles speech pathologist Morton Cooper, "is epidemic in this country. Anywhere from 25 to 50 percent of all Americans are misusing their speaking voice."

Cooper, who at $125 an hour is one of the highest-priced voice therapists in the country, says with no sign of nervousness that he would "starve if people ever learned to relax and speak naturally."

Although most of the "voice suicides" can be resuscitated, if your natural voice sounds like Jackie Kennedy or Michael Jackson and you're adjusting your pitch so it comes out like Eartha Kitt or Sylvester Stallone, you could be in trouble.

"If there's something about your voice you don't like," warns speech pathologist Virginia Jacobs, "make sure you're not going to do something even more artificial just to get a particular sound. It's not going to last very long and you may be doing something that will have negative effects for you."

Jacobs, also an assistant professor of Communications Science and Disorders at Towson State University, says her clients include "women who are truly smart, bright women, who are afraid to use their voices so that people perk up and listen to them. They have no authority to their voice and they don't know how to manage their voice when they're nervous."

One of Jacobs' clients was a singer who developed nodes on her vocal cords. "I was singing six nights a week," says Marian Koubek, 25, "and I would wake up hoarse. I had diminished range and no power and it was becoming increasingly difficult to sing and sing well."

Koubek, 20 at the time, had the nodes removed surgically and, after a period of therapy, resumed singing professionally. (She's now singing with Shooting Star, a Baltimore-area group, working part-time in a bank and studying at Towson for a degree in mass communications.)

Her therapy included learning relaxation and breathing techniques and how to speak correctly. The result: "I feel better about myself and my voice in general. That's the real key."

It is important, the experts stress, that people stay within their natural speaking range, but that doesn't mean having to speak in an unattractive tone, pitch or volume. "Anybody," asserts Cooper, "can have a beautiful voice if they just take some time and training to work on it. The voice is an instrument, like any other instrument."

Elizabeth Andaya, 43, a schoolteacher client of Cooper's, says her voice problem became so acute that she "just stopped talking because I knew I was hurting my throat but I didn't know what to do about it. I didn't talk at all for a period of two months."

When Andaya went to Cooper a few weeks ago, he told her she was speaking in too low a voice (and had been for more than 20 years). "He said I was only using my throat instead of my entire vocal mask, which includes the nose and mouth, and said that I needed to raise my pitch."

Cooper started her on a series of exercises. "I simply did what he told me," says Andaya, "and I've been talking ever since with no pain or discomfort. The longer I talk, the better it feels."

Although many people have difficulty accepting a "new" voice because the old one was such a part of their self-image, Andaya says she "loves" hers.

Minneapolis-based voice coach Rafi Dworsky says one of his biggest tasks with clients is to "help them come to terms with their expectations as they relate to their natural sound. It's almost always a psychological issue."

It is important to consider, notes Jacobs, that voice problems can be a purely mechanical matter. If a person has a breathing problem, such as breathing in the upper chest rather than from the diaphragm, his pitch may go up dramatically.

A possible solution: "Simply sighing," says Jacobs, "is a wonderful thing to do. Take a deep sigh, letting everything (including your shoulders) down for a minute. Then take a nice deep, refreshing breath. I think that's far better than going through the rigorous stuff that singers use . . . Relaxation is the key to good breathing, pitch, the whole thing."

Many voice problems come from nervousness and stress. "We're a very tense and hyperactive society," says Dworsky, "and while nobody ever dies from this, it concerns people more than anything else."

It isn't uncommon for overenthusiastic fans (remember the Redskins' playoff games and the Super Bowl?) to blow out their voices--like rock singers--from prolonged screaming, but it doesn't have to be that way. Says Dworsky, "There's not a healthy way to scream, but there is a healthy way to make a BIG sound."

What if you don't have a problem and just want to change your voice? You've got all the equipment you need, says Jacobs. You just have to understand how to use your voice.

But despite all the talk of voice problems, there are people who want to keep them. Jacobs tells of the time a doctor associate of hers was called to examine singer Carol Channing.

"Channing said, in that voice of hers, 'I'd like . . .' and the doctor said, 'Yes, Miss Channing, you do have voice nodules. Would you like me to set up an appointment and we'll have them removed?'

" 'Oh, no,' she rasped, 'I just wanted to make sure that they were still there.' "