The Voice of Business, once a harmonious chorus, is beginning to sound a bit discordant.

The small voices no longer want to keep tune with the large.

Leading the disharmony is Arthur Letitt Jr., the youthful chairman of the American Stock Exchange (Amex) and never one to sit silently by. He has been composing a few rousing songs that feature the smaller members of business' ensemble.

In his 18 months as Amex maestro, Levitt has become a national cheerleader for small- and medium-range businesses. Debunking the common impression that business sings a single song, Levitt has been going from coast to coast, making the point that there are at least two business chants-one for the big corporation and another for the underdog.

Often Levitt's themes sound very smiliar to the plaintive melodies of large corporations, stressing too musch regulation, too much taxation, too little government support of trade and too little public understanding of business' role. The difference, says Levitt, is that for big firms these issues represent merely important concerns while for small firms they mean life or death.

Take, for instance, the problem of raising money, which all businessmen have been griping is in less-than-abundant supply. "It's the second most important problem we have after inflation", said Levitt, in town this week for a national conference on medium-size companies. "Sure, big business talks about it, too. But at a price, firms like Du Pont and General Electric can raise captial. At no price can some small firms raise money today."

The problems of small have become a preoccupation for Levitt. The reason is hardly coincidental, given the job he has to do. A stronger small business sector will mean a stronger Amex, which has long been the little brother to the New York Stock Exchange where the shares of America's largest corporations trade.

Levitt's predecessor, Paul Kolton, did much to move the Amex into the active new markets of the 1970s, opening trading in stock options and futures contracts on the Amex while the NYSE balked at such expansion. But the outgoing, aggresssive Levitt has gone even further by taking the Amex show on the road.

He travels periodically to major U.S. cities-and to Europe in the fall-to listen to Amex members, scout for prospective ones, and introduce both to potential investors. In addition, he has become a regular visitor to the White House. Every six weeks or so, he meets with President Carter of Vice President Mondale and brings with him the chairmen of 20 or 30 small companies.

His purpose is clear: to build political channels for small business interests, separate from the deep and smooth channels already dug by such blue-chip groups as the Business Roundtable. The leaders of the big firms, Levitt has said, do not speak for all business.

"For too long, the voice of the midrange companies was muted while the corporate giants and the big labor organizations-those who could afford the lawyers and the lobbyists - was the voice that was heeded, was taken as the voice of business and of other economic interests," Levitt said in a speech yesterday.

"Never mind," he added, "that smaller enterprises account for 48 percent of the nation's non-form gross national product and 55 percent of our employment. And that they accounted for two-thirds of the 9 million new jobs created between 1969 and 1976."

As an example of where business interests sometimes diverge, Levitt recalled in an interview last year's congressional tax battle. The Roundtable favoured further depreciation allowances and elimination of taxation of dividends, he said, but small business spokesmen pressed for reductions in the capital gains tax.

Federal officials, too, often forget their regulations cut unevenly across the business sector, Levitt said. The Securities and Exchange Commissio, for instance, recently issued a rule requiring companies listed on stock exchanges to have audit committess made up of outside directors.

This was obviously a rule, Levitt said, whcih small firms would have more difficulty complying with than large companies since small firms have a more difficult time finding outside directors. As a result, Amex has drafted its own rule permitting small firms some relief in meeting the SEC requirement.00