Polaroid Corp. unveiled an experimental new stereophonic sound system today to go along with its beleaguered Polavision instant movies introduced last year.

The company also introduced a new, sharper film for its SX-70 camera at its annual stockholders meeting today.

The company's 1978 sales were up 30 percent to nearly $1.4 billion, and earnings per share climbed 28 percent to $3.60 despite sales problems with Polavision its first year on the market.

The new cassette movie system was poorly received, according to Wall Street analysts, many of whom were sour on the product from the beginning.

The cassette system accounted for a large part of the company's inventory problem, according to Polaroid Chairman Edwin Land, who demonstrated the new sound system today.

However, company spokesman Donald Derry said Polaroid has been "quite satisfied with (Polavision) sales since it went on the marketplace." There are no current projections on when the new laboratory model sound-movie system, designed to be used along with the current system without modification, will go into production.

Land said the new SX-70 film will produce a fully developed color photograph in about one minute, clearer and sharper than before, and can be seen developing almost immediately after it emerges from the camera.

This year's first-quarter earnings rose 18 percent to $17.1 million (52 cents a share) from $14.5 million (44 cents) a year earlier, as sales increased by 10 percent to $264.9 million from $240.7 million.

That report was below projections, and the company's stock has dropped in recent weeks.

Nevertheless, company President William J. McCune Jr. said "1978 was an extraordinary year for Polaroid by almost any standard.

"We do not expect to continue to grow at this accelerated rate, but we are indeed planning for continued growth," he said, adding that two of three of all instant cameras sold at retail in the United States were produced by Polaroid and the company's One-Step was again the best-selling camera in the nation.