The melody floated loud and clear from the capital's little amusement park, far from the battlefields that have made El Salvador important to the United States.

"Yankee Doodle went to town," the speakers played as a few Salvadoran children careened about in bumper cars, "Yankee Doodle dandy."

Just off the sidewalk where their parents were waiting in the afternoon heat, and only a few yards from the taco stand where some had snacked, a bored teen-age girl sold T-shirts from a little kiosk. Those for the children had U.S. cartoon characters. Those for the rest of the customers had the profile of a soldier with an inscription, written in English in bold, black letters:

"No to communism. We want peace."

RESIDENTS OF the capital say things have been getting more peaceful here over the last year. Even as the civil war intensified in the countryside, doubling the number of deaths in the Salvadoran Army, the streets of San Salvador seem to have become safer and livelier.

The change is relative. Men still are dragged from their homes or kidnaped in the street by armed gangs. As before, sometimes those who disappear turn up in custody of the National Police, under interrogation for suspected subversive activities. Other times they turn up dead, a bullet through the head.

Nevertheless, the atmosphere appears less threatening and more San Salvadorans are going out at night. Franciso Jose Guerrero, minister of the presidency, said recently that some of his friends who fled to Miami are beginning to return. In the anticipation that this will last, one couple has started a fancy new restaurant, Estro Harmonico. Diners willing to put up the stiff prices can enjoy a Central American rarity, real Bearnaise sauce.

About 30 persons, most of them young and well dressed, turned up at an opening for three Argentine painters the other night at Donatella Ferracuti's new gallery in one of the capital's fanciest neighborhoods. Parking was hard to find because dinnertime business was booming at restaurants on either side.

DESPITE THE WAR in its fourth year, San Salvador seems to have goods and food aplenty and, to a surprising degree, people with the money to buy them. One leftist political scientist suggested the ready cash is deceptive, that it comes less from production than from U.S. military and economic aid churning through the economy.

Whatever the source, the money often is spent quickly on consumer goods such as stereo sets and television recording machines. Flights arriving at the international airport from Miami or Los Angeles produce long lines at the customs counter.

That is, unless one can arrange "VIP treatment." This, a Salvadoran explained, can consist of holding some money inside the suitcase while the customs agent extends his arms under the contents from the other side. When he gets his hands to the traveler's side, the money changes owners, and the hands slide back under the now-exempt goods.

In another version seen recently, a passenger with six gold-colored watches spread them out on the counter for inspection by a customs agent. The traveler, with a chic woman at his side, looked quickly up at the agent, then separated one watch from the others. The inspector picked up the single watch, the passenger picked up the rest and everybody seemed happy.

CORRUPTION and even the civil war seem mild evils compared to the possession of two teen-age sisters by a devil, reported earlier this month in the town of Soyapango. The San Salvador newspaper El Diario de Hoy, in a full page of article and photos, declared:

"Some say it is a diabolical pact, others that it is a case of parapsychology. One thing is certain, though. It has awakened enormous curiosity, and the two girls have felt no relief even though an exorcism has been performed on them."

Readers were concerned for several days. But then a parapsychologist, Efrain Barraza Estrada, wrote in to ressure them that the Soyapango sisters were not the victims of the devil, rather of their own "hysterical personality" and the "collective hysteria" of their fellow townspeople.

"If there were demonic possession, she would turn her head 180 degrees, speak other languages . . ., she would move objects from a distance, and so on," he said. "There is no sign of this kind."