Despite the approach this week of the first top-level Soviet-American meeting since the Afghanistan invasion, detent's steady unraveling here has cloaked the entire foreign diplomatic community in deep pessimism about the future of the superpower's relations.

There appears to be little hope among foreign sources or the Soviets themselves that any substantive improvement in relations can be achieved during the Friday sessions in Vienna between Soviet Foreign Minister Andrei Gromyko and new Secretary of State Edmund Muskie.

The Kremlin has made clear that it is absolutely committed to preserving a Marxist government in Kabul and that it is not subject to any negotiation or pressure from Washington.

"The fate of the SALT II treaty and the American Olympics boycott are just not on the same level," one veternan foreign envoy said last week. "It is difficult to see how the two sides can proceed."

The stiff economic and cultural exchagne retaliation led by the United States and condemnation of the invasion by Moslem states and most of the rest of the world have startled and may deeply worry the Kremlin. But it appears that the best hope here is that Vienna will lead to more talks at at later date.

Those seeking signs of willingness by Moscow to begin the painful, protracted process of resuming dialogue with Washington found such an indication in the final paragraph of a lengthy Tass account today denouncing President Carter's speech last week in Philadelphia.

"What is needed in order to have normal relations is not demagogical utterances, but a sober approach, a frank and honest dialogue," asserted the dispatch, carried by major newspapers today.

This is the first time in months the Kremlin has hinted it thinks the Carter adminstration is capable of taking "a sober approach."

But it is the departure of Cyrus Vance as secreatry of state that may have given the Soviets an opening gambit to improve realtions. A Pravda commentator today described Muskie as "an experienced political figure" who could "bring some elements of soberness" to his job.

Against these new thin reeds is a powerful stream of Kremlin statements, actions and attitudes since the Afghanistan intervention that add up to a picture of Soviet intransigence. In recent talks in France, Gromyko gave no ground on the Afghan issue despite Soviet interst in using Paris to maneuver against Washington's European allies. Moscow also has just finished drawing lines within the Communist camp between supporters and detractors, such as the Yugoslavs and the Italian Communist Party.

One Soviet insider said recently he is convinced that the Politburo and President Leonid Brezhnev have given up any thought of rebuilding bilateral understanding until after the presidential elections. "They have given up on Carter for now." The comment tells much about Soviet perspectives, which exclude any self-recrimination for the Kremlin's own actions.

The myth of blamelessness has become reality here, despite the grave dangers it raises of superpower miscalculation. While some analysts cited the Tass dispatch as a possible sign of new flexibility, it generally attacked Carter in ways that underscore the tough Soviet attitude.

"The analysis of Soviet-American relations in Carter's speech is based on a distorted version of events in Afghanistan," the Tassa dispatch asserted. "The timely Soviet assistance to the Afghan people in rebuffing aggression from outside frustrated the plans for turning that country into an anti-Soviet bridgehead of U.S. imperialism.

"Carter demagogically asks when the Soviet Union will 'alter its conduct' to 'clear the way to improved relations.' It would be reasonable, however, to ask the president when the U.S. and its allies are going to stop the undeclared war on the Afghan people and the lawful government of Afghanistan, to end the policy of brazen interference in the internal affairs of sovereign states."

It concluded: "Carter talks about concern for the preservation of an American military strength that is second to none. At the same time, he seeks to pass off as "aggressive intentions' the lawful measure the Soviet Union has to take in order to consolidate its defenses.

"On the whole, the president's speech abounds in attacks and rhetoric hostile to the Soviet Union. And in the election year, this is precisely the course the president and one of his White House assistants try to conduct with the U.S.S.R. and other socialist countries."

The Tass account also highlights a steady Soviet move to cast the Afghanistan invasion in terms of its own defense instead of answering a request from an embattled Marxist neighbor. It is believed here that the Gromyko line in Vienna will include similar themes.

For U.S. diplomats here, the Soviet intransigence presents baffling problems. While U.S. Ambassador THOMAS J. Watson Jr. will join Muskie in Vienna, it appears that his own commitment to arms control rather than new ideas on how the United States can deal with the Soviet brings him to the meeting.

U.S. diplomatic contacts continue with the Ministry of Foreign Affairs down the street from the U.S. chancery and elsewhere. But the Soviets have dug in so deep on Afghansitan, the upgrading of NATO arms, the Iranian hostage crisis, and other issues that conversations on these matters seem fruitless. "Relations are cordial and correct and the Russians express happiness at seeing Americans," one source said, "but the important subjects are just not areas of constructive conversation."

Informed sources say the chill has extended to the Tass agency, the state radio-television monopoly and the important weekly Literaturnaya Gazeta which in recent weekshave quietly refused to accept the daily White House and State Department news summaries provided free for many years by the U.S. Embassy. The reason for the refusal, the sources said, was that some Soviet officials have objected to the alleged anti-Soviet propaganda contained in the handouts. About 130 organizations and officials elsewhere in the hierarchy continue to receive the mimeographed sheets.

Despite these gloomy and galling developments, no one has ruled out the possibility that behind its glowering mask, the Kremlin is preparing another face to show the world. Gromyko, and probably Brezhnev, will attend the 25th anniversary of the Warsaw Pace in a few days, and it is possible that they will use the occasion to unveil new disarmament proposals. But in the present climate, few can be found here who are willing to bet on it.