Forgotten operas can be a joy to hear -- sometimes

Whereas it is always pleasant to reencounter a favorite opera, there is a side of many an operagoer curious about the unknown or the forgotten. Sometimes we discover that the forgotten was actually best left that way. Other times, the encounter reveals something stirring, beautiful, and worthy of rehearing, or even staging.

Such a case has been made for Rossini's Semiramide across the years. Dame Joan Sutherland and Marilyn Horne brought it back to life in the '60s for such opera companies as Sarah Caldwell's in Boston, and the work was recorded for London Records in truncated form.

Miss Horne has been undaunted in her championing of the work. Her labors led to a wildly acclaimed staging in Aix-en-Provence a few seasons ago which was also seen in San Francisco last year. On both occasions, the title role was sung by Montserrat Caballe, and the role of Arsace by Miss Horne.

The Metropolitan Opera is evidently not interested in such lesser-known works. (The company has a history of denying its audiences the legendary vocal duo of Horne and Sutherland, and for ignoring the great bel canto works - those unique showcases created for great singers and great singing - even when it has had definitive singers at its disposal.) Miss Horne's interest in having New York audiences hear her in these roles led to the Gala Rossini Opera Festival series put on by Carnegie Hall, with ''Semiramide'' the most recent offering.

The opera was clocked at nearly 4 1/2 hours, with one short intermission. It is opera seria in full regalia - that curious form of operatic work in which the characters stop at every possible opportunity to sing of their specific emotions in lofty, lengthy arias. A composer of genius can make such a form work - as Mozart proved in ''Idomeneo'' and ''La Clemenza di Tito,'' and as Rossini amply shows in ''Semiramide.''

Since the opera is also a lyric bel canto product, the emphasis is not only on the expression of sentiments and feelings, but on vocal exhibition as well. Rossini was convinced that musical insight and vocal exhibitionism could go hand in hand to heighten any of his dramatic intentions.

And of course he was right. This opera is stuffed with superb arias that test every aspect of a singer's technique and art and give audiences an unstinting volley of superb melodies. Each aria furthers the story slightly as it airs the moods of that character's moment. The story tells a tale of love, deception, murder, mayhem, and matricide - a typical opera plot!

Miss Horne was, not surprisingly, magnificent. From softest floated tone to fullest raging roulade, she was in well nigh perfect control of her instrument and of the way the music needed to be presented to achieve its emotional effect.

Samuel Ramey revealed a facility to his bass-baritone which one encounters rarely, particularly in one who also makes such an impressive ''Mefistofele'' and ''Attila.'' He imbued all his music with theatrical force. Douglas Ahlstedt has made tremendous strides since he left the Met in the mid-'70s, but his lightweight tenor sounded strained in the upper reaches of the role.

The news event of the night was the last-minute replacement of Miss Caballe with the young American soprano June Anderson, who fared well, if not really yet in the league with Miss Horne. Miss Anderson's potential was constantly on view - a full-fledged coloratura with blazing top and a real sense of musical style. Also audible were her all-too-notable rough edges: If these are given time to be smoothed out, she will be a force to reckon with in a few years. If not, the loss would be a tragedy.

Henry Lewis was subbing on a week's notice for conductor Jesus Lopez-Cobos. He led the score with firm purpose, fine support for his singers, and a sense that he found the music at hand eventful and meaningful.

The final work in this Gala Rossini Opera Festival will feature ''Tancredi,'' with Miss Horne and Ileana Cotrubas, on May 22. Rare Strauss opera

Another little-known opera, Richard Strauss's Guntram, was conductor Eve Queler's preoccupation last week in Carnegie as the second offering in her Opera Orchestra of New York season. The work is of less consequence than ''Semiramide, '' but it is surely worth having a chance to experience Strauss's first opera.

The title role is for a heldentenor. The opera owes a good deal to Wagner's ''Tannhauser'' in plot direction. It owes much musically to Wagner as well, though when the heroine, Freihild, is finally given a scene to herself, we suddenly hear the Strauss that blossomed in ''Salome,'' ''Ariadne auf Naxos,'' ''Die Frau Ohne Schatten,'' and ''Arabella.'' Other moments foreshadowed other Straussian skills, and surely it would be worth having ''Guntram'' on records. But whereas ''Semiramide'' deserves an operatic airing in this city, ''Guntram'' need not concern our opera houses.

Miss Queler led the score with her accustomed professionalism, if without a real feel for the idiom that only in-depth familiarity with the Strauss canon would permit.

Reiner Goldberg, hailed in Europe as the newest heldentenor, was making his American debut in the title role. He had audible problems sustaining the wickedly high-lying role, and nothing in the voice suggests that he will be up to the title role of ''Siegfried'' in the new Sir Georg Solti-led ''Ring'' at Bayreuth this summer.

As Freihild, Ilona Tokody revealed a pleasing lyric soprano pushed fully beyond its resources in a role that could have been written for that preeminent Strauss soprano Leonie Rysanek. The rest of the singing remained staunchly in the acceptable range. 'Pelleas' and 'Hoffmann'm

A brief word is in order about two French opera revivals at the Metropolitan this month. Debussy's Pelleas et Melisande may never gain status as a beloved opera, but the Met is noble in presenting it every so often so that those who dearly love the work - this writer included - can experience its unique magic and utter beauty.

James Levine conducted superbly. Jose van Dam is probably the finest Golaud of the day and one of the greats of all time in this role. Jeannette Pilou and Dale Duesing sang the title roles, and Jerome Hines was Arkel (the night I attended, Richard Vernon took over in the last two acts). On the gossamer Desmond Heeley sets, Fabrizio Melano's direction looks at best dutiful and often confusing. It may not have been the finest revival, but it never slumped below routine, and any chance to hear this work in New York is welcome.

The stunning Otto Schenk production of Offenbach's Les Contes d'Hoffmann is now in its first revival, and it has lost some of its edge. Happily, Julius Rudel (who will conduct the radio broadcast of Feb. 5 - check local listings) maintains a high level as conductor, breathing fire and life into the score.

Catherine Malfi-tano and James Morris gave performances that amply demonstrated why they should have been included in the first cast of the new production last year. Tatiana Troyanos, the only holdover from the new production, was in poor form. Kenneth Riegel in the title role and Gianna Rolandi, the new Olympia, were overparted. But the production remains larger than life in its fantasy and magic, which is as it must be for this work.