It is rarely a good idea for operatic singers to record popular songs, though the temptation is frequent and too seldom avoided. One recent and striking exception is Luciano Pavarotti's new collection of Italian popular songs: "Volare" (London 421 052-2). These songs often have a strong affinity to operatic style; some (such as Mascagni's "Serenata") are by operatic composers, and Henry Mancini's orchestral arrangements emphasize the operatic dimension. Pavarotti is in excellent voice for the 16 selections on this disc, and it is recommended with two reservations: Texts and translations should have been included in the package and Domenico Modugno's beautifully proportioned balance of recitative and aria in "Volare" is arbitrarily altered. Even humble pop tunes have their own integrity and it should have been respected.

An almost ideal matching of classical voices with popular material can be heard in the MCA recording of Rodgers and Hammerstein's "Carousel" (MCAD-6209). Produced by Thomas Z. Shepard, the best producer of Broadway musical recordings in the business, this brilliant disc establishes "Carousel's" status as a classic if anyone still doubts it. Samuel Ramey, a great Don Giovanni on television last week, is also a great Billy Bigelow, responding to every nuance in the soliloquy and "If I Loved You" with impressive tonal richness and dramatic presence. Maureen Forrester brings one of the century's great voices to "You'll Never Walk Alone" and "June Is Bustin' Out All Over." The two sopranos, coming out of the musical comedy tradition, are completely idiomatic. Barbara Cook is a definitive Julie Jordan, with a voice fully capable of matching Ramey's in duets, and Sarah Brightman brings a good voice and a vivid personality to the role of Carrie Pipperidge.

Brightman has the starring role in the London cast recording of her husband's "The Phantom of the Opera" (Polydor 831 273-2 Y-2, two discs). Like parts of Andrew Lloyd Webber's Requiem, this opus seems designed largely as a showcase for Brightman's voice, and it serves that purpose well. The composer is clever at imitating 19th-century styles of vocal and ballet music, but without the special effects of the stage production the music seems undernourished compared with such classics as "Carousel," "Sweeney Todd" or "My Fair Lady."

The latest fair lady on records is Kiri Te Kanawa, and whatever you may have thought about her work in "West Side Story" or "South Pacific," she seems well cast vocally, linguistically and temperamentally as Liza Doolittle in "My Fair Lady" (London 421200-2 LH). The competition is fierce, of course, and she does not erase memories of Julie Andrews, but she makes the role convincingly her own. Personally, I had more of a problem accepting Jeremy Irons as Professor Doolittle, not because there is anything wrong with his performance but because Rex Harrison (who could not sing nearly as well) has established so effectively the timbre of Doolittle's voice, the way he pronounces words and inflects a sentence. This reaction is clearly unfair, but I suspect it may not be uncommon. You are not likely ever to hear "On the Street Where You Live" sung better than it is sung by Jerry Hadley. John Mauceri conducts (as usual) brilliantly, and the greater capacity of compact discs (more than 70 minutes) allows the inclusion of material left out of the original cast album.

Operatic voices are not quite necessary for most Gilbert and Sullivan roles, but better voices unquestionably make the music more effective, as the Washington Opera demonstrated this season with its brilliant "Ruddigore." A semi-souvenir of that production can be heard on a new MCA recording (MCAD2-11010, two CDs) that uses different voices but the same recently reconstructed score that was heard in the Eisenhower Theater.

The new recording, performed by the New Sadler's Wells Opera, is titled "Ruddygore," using the spelling of Gilbert and Sullivan's original working title. That spelling was modified because tender Victorian sensibilities saw it as borderline obscenity. The restored original spelling is appropriate because this is the first recording of the restored original score. The D'Oyly Carte recordings, based on scores that were revised in the 1920s, are significantly different from this one, particularly in the Act 2 finale, and the restored score in this new recording is considerably better. The performance is excellent, though it does not quite reach the Washington Opera's level in the duet, "I Once Was a Very Abandoned Person." But to do justice to that performance (one of the highlights of this season) a video recording would be essential.

MCA, obviously getting seriously involved in musical theater recordings, also has a first-class disc of highlights from "The Mikado" in the English National Opera production (MCAD-6215).

In its midpriced "Victor Opera Series," RCA is rereleasing on CD some of the best opera recordings in its rich archives. The "Barber of Seville," conducted by Erich Leinsdorf and starring Robert Merrill, Roberta Peters, Cesare Valletti and Giorgio Tozzi, has been one of the best recordings of this opera (for my taste, the uncontested best) since its first appearance in 1959. Its new edition (6505-2-RG, three CDs with libretto) reestablishes that status resoundingly.

Equally impressive is the 1953 "Cavalleria Rusticana" (6510-2-RG, with libretto) with Zinka Milanov and Jussi Bjoerling joining Merrill. Milanov and Bjoerling, sometimes with Merrill and sometimes with Leonard Warren, usually with Renato Cellini conducting and Robert Shaw directing the chorus, made some recordings at the dawn of the LP era that have stood the test of time and should be brought into the CD era. This powerful "Cavalleria" should be a harbinger of great things to come.

Robert Shaw is the only member of that team still active, and he is nearing retirement as music director of the Atlanta Symphony Orchestra and Chorus. One of his valedictory recordings, Verdi's Requiem, recently issued by Telarc (CD 80152, two CDs with libretto), will serve as a suitable monument to his distinguished career. It contains a technically brilliant, deeply felt performance of the Requiem, recorded in Telarc's usual meticulously pure and detailed sound. All four soloists are impressive; tenor Jerry Hadley and bass Paul Plishka stand out. The second disc is filled out with five of Verdi's best operatic choruses, including "Gloria al Egitto" from "Aida," "Va, pensiero" from "Nabucco" and "Fuoco di gioia" from "Otello."

Meanwhile, other outstanding items in the Victor Opera Series include a "Rigoletto" conducted by Sir Georg Solti with Merrill, Anna Moffo and Alfredo Kraus (6506-2-RG, two discs with libretto), and a brilliant "Ernani" conducted by Thomas Schippers and starring Leontyne Price and Carlo Bergonzi (6503-2-RG, two discs with libretto).

Inevitably, EMI will reissue on CD the great "La Bohe`me" conducted by Sir Thomas Beecham that now graces its archives, and among the older reissues that will become the edition of choice for most opera lovers.

But until then, RCA's 1961 "La Bohe`me" (3969-2-RG, two discs with libretto) will do nicely. It has much better sound than the Beecham, and it is brightly performed by Anna Moffo, Richard Tucker and Merrill with Leinsdorf conducting.

Moffo faces even harder competition in "Lucia di Lammermoor," with a more dramatic performance by Callas and a more vocally spectacular performance by Sutherland already out on CD. Hers is nonetheless a fine "Lucia" (6504-2-RG, two discs with libretto), dramatically more convincing than Sutherland and vocally more secure than Callas. Carlo Bergonzi and Ezio Flagello are excellent in supporting roles.