The process of certifying El Salvador's "substantial progress" in human rights has now become stylized, with everyone coming forward on cue to sing a stale air.

The lead tenor, President Reagan, appeared last Friday to say, as expected, that that blood-drenched country is now marching toward the sunshine of democracy.

Its tragedy to him comes down to this: he told a group of high school students that the situation has been "distorted by a worldwide propaganda campaign from the Soviet Union via Cuba."

He was followed by two monotones of certified insensitivity from the State Department, which offered the well-known theme that things may not be good in El Salvador, but are much better.

Yes, innocent civilians are still being murdered, and, alas, by government forces, but at a much slower rate.

The body count, which was 800 a month in 1980, is down to 200 a month. It is morality by measure.

The baritone of the American ambassador to El Salvador, Deane Hinton, is heard. Last October, Hinton struck a discordant note. He attacked the murderous security forces of the country. He called them "right-wing gorillas" and told them to their faces that they were as much of a threat as left-wing guerrillas.

The Salvadoran establishment huffed and puffed about unwarranted interference by a "Roman proconsul." He was, more significantly, reprimanded by his superiors in the State Department.

A week before the expected certification, Hinton came out with the naked truth that condemns thousands more Salvadorans to war and death.

Said Hinton, "Surely, any president or any administration that thinks it would be a disaster if this country was taken over by a totalitarian Marxist regime is going to hesitate a long time and the evidence would have to be very strong before he decides not to certify."

There was, of course, never any chance that Reagan would not certify the government for further U.S. aid. He tipped his hand during his December trip to the region, where he memorably exonerated neighboring Guatemalan President Gen. Efrain Rios Montt, who stands on a pile of peasant corpses, of human rights violations.

After the soloists comes the chorus to sing a dirge for the country. It is exceptionally large this year. Some 17 delegations--members of Congress, doctors, educators, television stars, rock singers, health experts, lawyers--have made pre-certification visits to the "weary bourne" to hear at first hand the familiar stories of death, disappearance, dismemberment, torture and rape.

They have talked to army officers, civilian leaders, priests, prisoners, refugees, peasants, businessmen. They tell their stories, knowing it is a waste of breath as far as their own government is concerned.

The families of the four murdered American churchwomen are heard from. The long-delayed trial of the soldiers charged with the murders has struck a snag. New evidence has come to light. To deal with it in El Salvador's serpentine legal system will take four to five months. The women were killed in December, 1980.

At a news conference where Mary Travers of Peter Paul and Mary, and Mike Farrell, the beguiling B.J. Honeycutt of "M*A*S*H," tell of conversations with Salvadorans who have been terrorized by their own government, Rep. Edward F. Feighan (D-Ohio), says he will introduce a bill in the House to overturn the certification. It might pass the House, but never the Senate.

In all the predictable noises of this third certification, there is just one different sound. The AFL-CIO, which sponsored the first two, is balking. It is, in view of labor's record of Cold War solidarity with the administration in Latin America, a surprising and heartening development.

The AFL-CIO, which has been working with the Agency for International Development on land reform--with results that are hotly disputed--is enraged by the response of the Salvadoran justice system to the 1981 murders of three union workers, two Americans and one Salvadoran.

The union hired an ex-FBI agent to investigate. He found the killers, and the perpetrators.

One of the fingermen, a Lt. Rodolfo Lopez, a confederate of the notorious Roberto D'Aubuisson, was let go by a corrupt or timid judge--there is no other kind in El Salvador--on grounds of insufficient evidence. He was allowed to dye his hair, shave his mustache and put on eyeglasses before appearing before witnesses. Lopez has been returned to active duty.

William Dougherty of the American Institute for Free Labor Development is ready to testify against certification.

He admits that it looks as if labor was just trying to avenge the murders of its own people. He regrets that while thousands of Salvadorans have died--36,000 in the past four years--the deaths of seven Americans constitute the only leverage that outraged Americans have on their own government.