The small electronic hearing device that recently showed up in President Reagan's right ear has turned into an advertising bonanza for the country's hearing-aid industry, with manufacturers swamped by orders.

The Hearing Industries Association now estimates that sales will top a record one million this year, a 40 percent jump over the 1982 total. Experts say dollar volume, about $300 million last year, will increase proportionately.

Reagan's hearing aid is getting most of the credit because it showed millions of the elderly that hearing aids are not something to cause embarrassment, industry officials said.

"It was the single biggest thing to happen to this industry in 30 years," said Al Bruce, a spokesman for Starkey Labs Inc. of Minneapolis, the largest hearing-aid firm and the manufacturer of Reagan's custom-made $1,000 device.

Bruce says the boom began right after the news about Reagan's device broke on Sept. 7. "The next day, our switchboard was kind of inundated," he said. "The calls were coming in every half-a-minute."

More important for Starkey, sales also surged. The firm, which normally receives about 60 orders a day for the deluxe two-ounce "canal aid" that Reagan bought, has been selling about 120 units a day ever since.

Others in the industry report a similar response. "We were getting calls from people saying, 'I want one just like the president,' " said Lois White of the National Hearing Aid Society, a trade group representing hearing-aid professionals. "Suddenly, it became acceptable . . . People were saying to themselves, 'Nobody's going to think I'm senile just because I'm sticking that in my ear.' "

Ironically, Reagan's entry into the hearing-aid market comes at the same time administration appointees at the Federal Trade Commission have been working to scuttle a proposed rule to crack down on alleged industry abuses.

Top officials at the FTC's Bureau of Consumer Protection have recommended scrapping a staff proposal to require a 30-day trial period on hearing devices. The case has been on the desk of Commissioner George W. Douglas since April, but no action is expected any time soon, according to a Douglas aide.

The FTC staff has estimated that, if the rule is imposed, between 2 percent and 7 percent of hearing-aid purchasers would return them because they don't work.

Despite an industry code of ethics that requires such a tryout, "there is still substantial reason to believe there are many people who are getting hearing aids and aren't benefiting from them," said one FTC staffer.

At last word, Reagan isn't one of them. But, the staffer adds, "I don't think the president is going to have to worry about getting a free trial period."