HOW GOOD CAN a recorded cassette be? It was not so long ago that no one took the cassette very seriously as a medium for recording music. It was regarded as handy, to be sure, but the possibility of its every being able to match the sound quality of a first-rate disc, let alone open-reel tape, seemed remote at best. Things have changed.

In 1975 the Advent Corp., makers of speakers and cassette decks, came out with a series of carefully mastered cassettes on chromium dioxide tape, offering recordings from the Nonesuch and Connoisseur Society catalogues and a few original items. The sound of Jascha Horenstein's Mahler and Leslie Jones' Haydn was beyond anything we had heard in this medium before, and some listeners swore it was superior to that of the respective discs.

Mobile Fidelity Sound Lab, in its series of Original Master Recordings leased from RCA, Angel and London, produced a few outstanding cassettes, and the chrome cassette editions of various companies' digital recordings have been impressive. But most impressive of all, without question, is the In Sync line produced by Alan Silver -- cassettes so incredibly realistic, so wide open in tonal and dynamic range and so totally free of hiss or other background noise that even the best open-reel specimens may be measured against them -- and they get better with each successive release.

More than 20 years ago Silver created the Connoisseur Society, on which label he was the first to issue 12-inch stero discs playable at 45 rpm (something "aduiophile" companies in England and Japan have done only recently.) Eventually he converted his catalogue to the standard 33 1/3 speed for his own recordings and the material he leased from EMI's French company. A few years ago he abandoned his disc production altogether to concentrate on the In Sync cassettes.

At first Silver used chromium dioxide tape, and, like other purveyors of tape recordings, bought it in bulk so he could cut to fit particular recordings. Recently he switched to TDK's SA-X, a ferricobalt tape he describes as "Japan's answer to chromium dioxide." Among its advantages are lower print-through, more head room, greater usable dynamic range and virtual elimination of dropouts. (The labeling still reads, "chromium dioxide," since the label cards were printed in quantity some time ago.)

The mastering is done in "real time," and in preparing his new series of recordings by several fine American orchestras -- the Cincinnati Symphony under Susskind, Leonard Slatkin and Jerzy Semkow, the Minnesota Orchestra under Stanislaw Skrowaczewski and the Atlanta Symphony under Robert Shaw -- Silver has had the huge advantage of being able to work directly with Marc Aubort, the engineer who made these recordings in the first place.

Aubort and his associate, Joanna Nickrenz, make these recordings for Vox and that company's successor, the Moss Music Group, as far back as 1974, in the "QS" quadraphonic system. While no one likes to talk about 4-channel now -- many of the allegedly compatible discs were not really compatible for fully satisfactory two-channel playback -- having the four channels to work with was a definite advantage in remixing these recordings. The sumptuousness, depth and definition in the original master tapes are beyond anything suggested in the disc editions, and it has all been opened up with astounding brillance on the In Sync cassettes.

Those performances that impressed most strongly -- such as Skrowaczewski's Ravel cycle and Slatkin's Rachmanioff -- are even more dazzling in their appeal now, and some that seemed little more than competent on the Vox or Turn-about discs are so powerfully seductive in their new incarnations that they must be reevaluated on musical as well as sonic grounds. One of these is Susskind's Saint Louis recording of "The Planets," so brilliantly effective here that it may be regarded as one of the choice versions of the Holst showpiece in any medium. Another is Schippers' package of Strauss tone poems and opera excerpts.

The In Sync cassettes are available in a choice of Dolby B or C noise reduction treatment, and some in Dolby C on metal (TDK MA) as well as the ferricobalt. Since Silver is using TDK blank cassettes for his transfers, instead of bulk tape, there is a good deal of run-off time on some of them, and some listeners may find this a nuisance. However, it seems a small price to pay for sound of this quality, and C-90 tapes are offered for the same price as the C-60s.

These cassettes are not cheap. The store price is $17.98 each (discounts are available to mail-order purchasers). But this, after all, compares with the cost of many audiophile discs, and In Sync offers a generous and probably unique double guarantee: (a) free replacement of any defective cassette and (b) replacement of a user-damaged cassette for only $2. Program notes are not included, but a request form is, which purchasers can mail in to receive the free notes.

The 20 cassettes issued or announced so far are listed here. The six highest numbers (4117--4122) may not reach the shops till mid-April; one of them, Slatkin's coupling of Rachmaninoff's "Isle of the Dead" and "The Bells," is regarded by Silver as the finest sound he has yet achieved. As nearly all the performances are fully competitive with the best available on discs, this series constitutes quite a showcase for these supposedly "second-level" American orchestras. The performers are identified as follows:

(A) Cincinnati SO under Schippers

(B) Cincinnati SO under Susskind

(C) Saint Louis SO under Slatkin

(E) Saint Louis SO under Semkow

(F) Minnesota Orchestra under Skrowaczewski

(G) Altanta SO under Shaw

Bernstein: Symphonic Dances from "West Side Story"; Tchaikovsky: "Romeo and Juliet" (G) 4105

Gershwin: "Catfish Row" (Gershwin's own suite from "Porgy and Bess"); "An American in Paris"; Promenade (D) 4106

Handel/Harty: Waler Music and Royal Fireworks Suites (F) 4118

Hoist: "The Planets" (C) 4103

Mussorgsky/Ravel: "Pictures at an Exhibition"; Mussorgsky/Rimsky-Korsakov: "Night on Baid Mountain" (D) 4115

Prokofiev: "Alexander Nevsky" (with Claudine Carlson and chorus); "Lt. Kizheh" Suite (D) 4116

Rachmaninoff: "The Bells" (with soloists and chorus); "The Isle of the Dead" (D) 4121

Rachmaninoff: Symphonic Dances: Three Russian Songs for chorus and orchestra; Vocalise (D) 4117

Rachmaninoff: Symphonies Nos. 1, 2, 3 (D) 4110, 4107, 4109

Ravel: Bolero; La Valse; Pavane pour une infante defunle; Rapsodie espagnole (F) 4104

Ravel: "Daphnis et Choie" Suites 1 & 2; Alborada del gracioso; Menuet antique; Le Tombeau de Couperin (F) 4112

Ravel: "Mother Goose" (complete ballet version); Une Barque sur l'ocean; Fanfare for "L'Eventail de Jeanne"; Valses nobles et sentimentales (F) 4111

Rimsky-Korsakov: "Scheherazade" (E) 4114

Shoslakovich: Symphonics Nos. 1 & 9 (B) 4122

Strauss: "Don Juan"; "Till Eulenspiegel"; Salome's Dance; "Rosenkavalier" Waltzes (A) 4113

Stravinsky: "Firebird" Suite; Prokofiev: Scythian Suite (F) 4119

Wagner: "Meistersinger" & "Rienzi" Overtures; "Parsifal" -- Good Friday Spell; "Lobengrin" -- Preludes to Acts I & III; "Walkure" -- Ride of the Valkyries (E) 4120

Wagner: "Tannhauser" -- Overture and Venusberg Music (with chorus); "Tristan und Isolde" -- Prelude, Liebestod, Prelude to Act III (F) 4108

If you can't find these cassettes where you shop, you can get information and order from in Sync Laboratories Inc., 2211 Broadway, New York, N.Y. 10024.