Worse yet, there may have arisen a feeling that the strained sounds are in themselves dramatic: that hearing a voice pushed past its natural limits, and empathizing with the performer in extremis, adds an edge to operatic excitement. This is a decadent pleasure: Each incremental step away from vocal values works decay on the art that was the original source of the pleasure.

The result is a generation of audiences and critics less prepared to listen for the finer points of singing - and a generation of singers less prepared to heed, or even notice, the warning signs until serious remedial work is required and their original ease of singing probably irrecoverable.

Lyric sopranos like Ileana Cotrubas and Mariana Nicolesco became shrill and labored-sounding just as they entered what should have been the period of their best work. In Alan Titus, J. Patrick Raftery, Brent Ellis and Richard Stilwell, America has four young baritones who still sing creditably, but not with the tonal suavity they began with. Three potential heldentenors whose voices have been compromised since they appeared on the scene are Rene Kollo, Siegfried Jerusalem and especially Peter Hofmann, who sounded at the end of his tether at the Met last year -and yet is the single most crucial tenor ingredient of the house's Wagner efforts for several seasons to come. The list of sopranos in whom one can hear an echo of the problems Beverly Sills developed is too long to name.

And an astonishing number of singers in midcareer, some of them guided by equally astonishing advice from influential conductors and operatic power-brokers, are avidly courting disaster with an assault on roles manifestly too heavy for their voices. (Operatic roles are spoken of as ''heavy'' or ''light'' on the basis of the volume, stamina and kind of vocal activity required.) Katia Ricciarelli, 15 years ago a lovely lyric soprano, has pushed her shaky technique through the heaviest roles in the Italian repertory during her years on the big-house circuit: Tosca, Aida, even Turandot. She wobbles, her high notes are often quite ugly, her middle range often hoarse and weak. Will she retrench? No. She is now announced to sing Norma, a role combining ferocity of utterance with the most stringent demands on sheer vocal accomplishment. The splendid Rossini and Mozart tenor Francisco Araiza has declared his intention to advance to the strenuous parts of Don Carlos, Alvaro in ''Forza,'' Lohengrin and other Wagner heroes, Andrea Chenier, Canio in ''Pagliacci.'' The lyric soprano Catherine Malfitano has taken on Berg's heavily orchestrated ''Lulu'' in Munich and will soon do the taxing, marathon part of Madame Butterfly. Her Met performances last year suggest that she is already beginning to pay a price. Carol Vaness, whose strong, slender, thrusting voice has hit a series of bull's-eyes in Mozart and Handel at the Met, has decided to plunge into the broader, fuller outpourings of Verdi. She has done two middleweight Verdi operas with equivocal results; undaunted, she will try on a few more for size. Jose Carreras, whose beautiful lyric tenor is already in widely noted and serious vocal decline, imposes on the world's opera houses choices between such heavy tenor leads as those of ''Forza,'' ''Andrea Chenier'' or ''Pagliacci'' as a condition of his appearance.

The singers who should be filling those heavier roles, meanwhile, are among the most prone to wear themselves ragged at an early age (following the Callas model of excitement through abuse of the voice). Anita Cerquetti and Elena Suliotis are two famous examples of dramatic sopranos who vanished altogether after a couple of high-wire seasons; others continue their careers on a lower level. Among sopranos alone, Olivia Stapp, Carol Neblett, Marilyn Zschau, Marisa Galvany, Stefka Evstatieva and Sylvia Sass are all valuable players whose careers seem to have crested without ever reaching the level that was hoped for them.

All this cannot help but sadden and anger an observer who loves the human voice and respects the art of singing. T HE ENVIRONMENT that used to promote vocal longevity can probably never be fully regained. Promising young singers were once identified at an early age, taken on as proteges by celebrated teachers, supported by patrons and trained daily for years. That sort of thing has largely vanished. In an earlier age, the difficulty of travel made singers plant themselves in one spot for the bulk of an operatic season; they had, without even thinking about it, a stability of life style and artistic surroundings that is utterly lost to their jet-age successors - a crucial factor during the years when a young singer is learning from role models among his older colleagues, from repeated ventures in a theater where everything doesn't stand or fall on how it goes this one night.

An aid to vocal survival that ought to be recoverable is some degree of flexible planning on the part of opera houses. As late as the early 1960's, Rudolf Bing used to plan each Met season while on the spring tour of the previous one. Today, the pool of world-class singers is small and shrinking, while the number of houses bidding on them has grown, creating a spiraling demand for advance commitments. Even second-rank singers are hired three, four and five years ahead by all the major houses. And once roles are chosen, designers and directors engaged, it becomes exponentially more difficult for the management to reshuffle its singers or its schedule -or for a singer to say, ''I am no longer comfortable with/ should never have accepted/ need more time to prepare that part.''