Every morning and every night a staccato burst of recorded gunfire, martial music and Marxist rhetoric punctuated by the whoop of electronic interference are familiar sounds here as the guerrillas take to the airwaves.

People listen because they sympathize, or because they are curious, or in many cases just to find out something about what is going on in the inaccessible northern and eastern parts of the country dominated by the insurgents.

Then, through the crackling noise that blocked a recent broadcast, many people thought they heard an insurgent report that the oldest and most famous rebel leader, Salvador Cayetano Carpio, was killed in action. Local newspapers reported his death. Foreign correspondents tried frantically to check it out.

But Carpio was not dead.

Instead, he was the victim of something he had denounced on the air only a week before, a new antiguerrilla radio station.

"Using the same frequencies, schedule and style of the revolutionary stations," said Carpio, "it is spreading absolutely harebrained lies about the glorious revolutionary struggle."

In fact a major electronic battle is running side-by-side with this region's shooting wars as clandestine transmitters and carefully plotted propaganda increasingly are used to supplement, amplify and often mystify what each side is doing with its guns and grenades.

In Central America, where many are illiterate, radio is the mass medium with the broadest reach and the biggest impact on peasants whose major contact with the contemporary world may be what comes to them through cheap transistors.

Revolutionaries were quick to see the possibilities. Nicaragua's Sandinistas were on the air long before they were in the capital and their Voice of Nicaragua and Radio Sandino now saturate a broad band of frequencies reaching out all over the region.

Those on the other side are just as accustomed to the advantages of underground media. As long ago as 1954, as has been documented in several recent studies, Central Intelligence Agency-financed broadcasts over a clandestine Voice of Liberation played a crucial role in ousting the leftist Guatemalan government of Jacobo Arbenz.

Prepared by E. Howard Hunt and David Atlee Phillips, among others, the 1954 broadcasts undermined Arbenz's support in his own armed forces by spreading word he was getting ready to disband his Army, then helped convince even Arbenz that a massive force was moving on the capital instead of the rag-tag group of irrgulars that really existed.

Now, the "counterrevolution" fights back at the Sandinistas with Radio 15 de Septiembre, spreading word of Sandinista atrocities that, true or not, guickly work themselves into the mythology of Miskito Indians and other groups opposed to Managua's Marxist leaders.

Another station called The Voice of Sandino is also beaming its message to Managua.

Its reported message on Nov. 5, for instance, was that the feuding factions of the Miskitos had agreed to fight together against the Sandinistas "in order to paticipate in the true liberation of the Nicaraguan people."

In El Salvador, the guerrillas have two stations in regular operation, run by two factions in two different parts of the country.

Radio Venceremos (which means "We will win") has been broadcasting the message of the guerrilla front as viewed by the People's Revolutionary Army faction on an occasional basis since early 1980 and regularly since 1981. Radio Farabundo Marti (named for the 1930s Salvadoran Communist admired by the rebels) came on the air more recently and with a weaker short-wave signal that broadcasts variations on the front's theme as seen by the more hard-line Popular Liberation Forces. A few weeks ago, Venceremos added regular FM broadcasts to its dimly heard short-wave bands.

Graffiti on bombed-out bridges tell you where to tune in, but nobody tells you where to find the transmitters.

Despite vows to knock them out, the U.S.-backed Salvadoran Army has had little success forcing them off the air. Washington frequently charges, but has never publicly proved, that the main transmitter is next door in revolutionary Nicaragua.

The new short-wave weapon against the rebel radios went into action last month. As clandestine any mysterious as they are, it calls itself Radio Orientacion (Orientation) but is getting to be known on the street as Radio Mentiremos ("We will lie").

The guerrillas have told reporters they think the new station is broadcasting from somewhere in the wealthy neighborhood of Escalon de San Franciso in the capital, probably run by people associated with the right-wing political party of Assembly President Roberto D'Aubuisson. The guerrillas also guess that the money for the station comes from the CIA.

Carpio called it part of the Reagan administration's "demented warmongering plans" and "an instrument of the Salvadoran government's fascist psychological warfare."

Spokesmen for the government say they know nothing about the operation.

U.S. Embassy officials, asked simply who they think is running the new radio station, answer with "no comments," then explain in private that standard guidance is not to comment in any way on intelligence activities.

Does that mean it is connected with U.S. intelligence?

"No comment," they said.