A plea for correct opera casting and three recordings that didn't

It's hardly a secret that casting an opera these days is problematic: Too often pivotal roles demand voices that do not exist today. So compromises have to be made.

The same holds true in the recording studio. One might think that with the technology of microphones, sound enhancement, and so forth, small voices could be made to sound large. However, microphones do not lie. A small voice sounds like a small voice. An essentially lightweight voice cannot be disguised to sound ''larger'' by merely being closer to a microphone.

In the three operas in question here, the title role requires a very specific type of voice that still exists today. However, the recording companies do not seem to feel some of these singers are marketable commodities. So they have been denied the chance to record in favor of some star singer, even when that star lacks the fundamental requirement of the particular role.

A case in point is a recent ''Tosca'' (Angel digital DSBX-3919). Renata Scotto sings Puccini's heroine; Placido Domingo is the Cavaradossi; and Renato Bruson, the Scarpia. James Levine leads the Philharmonia Orchestra. Miss Scotto is a fine Puccini singer; in ''Boheme'' or ''Butterfly,'' she has proved incomparable. But Tosca's wide range of emotion demands a large voice capable of an unusually wide range of dynamic shifts from the very quiet pianissimo to the full-throated forte.

On this recording, Miss Scotto's penetrating, lyric soprano reveals the effects of constantly singing in heftier repertoire. Her slender voice was never meant to tackle such dramatic soprano roles as Norma, Gioconda, and Lady Macbeth. Miss Scotto can no longer swell from a pianissimo to a full-throated forte without a severe wobble disrupting the vocal gesture. The overall feeling here is one of good intentions executed with a now-virtually unmanageable instrument. Considering she is one the day's finest lyric singing-actresses, this career shift has been a lamentable decision.

Mr. Domingo is in strident voice here. Mr. Bruson's lightweight baritone is incapable of communicating the overpowering menace and sinister essence of Scarpia. James Levine magnficently conducts a grandiose, electrifying account of Puccini's opulent, thrustful score. It is a grand irony of this set that the conducting should be outstanding while the casting is not at all right. The sound throughout the set is all one could ask it to be. A footnote of interest: the Jailer is sung by noted violin virtuoso Itzhak Perlman - and very well sung, too.

Ponchielli's ''La Gioconda'' was once relatively easy to cast. It is a grand spectacle opera that pits revenge, passion, and filial duty against the backdrop of a bloody, dictatorial Venice. The title role requires a voice of size, beauty , and stamina. Monsterrat Caballe has been branching out - none too successfully - from her bel canto roots to the full-fledged dramatic soprano roles, and Gioconda might seem ideal for her. But her performance here (London digital LDR- 73005) finds the voice underweight for the musical demands. There are many handsome moments; Miss Caballe often sings with beauty and some grace; but as the voice needs power, it assumes a stridency that disfigures the line and cumulative impact.

Luciano Pavarotti does not possess the vocal weight to sing Enzo. Yet here he manages to fill the music with ardor and fervency. Agnes Baltsa, another lightweight singer, convinces yet again in a role she would have trouble sustaining in an opera house. Nevertheless, few mezzos today - even those with the requisite vocal equipment - can project the varied moods and emotions with the acumen Miss Baltsa consistently manifests in her work.

Sherrill Milnes has often sung Barnaba on stage. Here he is in generally good voice, offering the only real touch of traditional authenticity in the performance. Nicola Ghiaurov is lavishly cast as Alvise. In all, it is a good recording for the '80s, without erasing memories of most other past recordings of the opera. The true weakness is Bruno Bartoletti's erratic, unsympathetic conducting. He rushes where one should luxuriate, dawdles where haste is required. And he generally makes musical decisions that have no justification in the music. However, the National Philharmonic Orchestra plays handsomely for him.

Puccini's ''Turandot'' is the province of dramatic sopranos on stage. In the studio, Joan Sutherland made an impressive stab at the part, but her career began as a Wagnerian singer, and the instrument itself possesses weight and size. A few years back, Angel records issued a ''Turandot'' with Miss Caballe, Mirella Freni, Jose Carreras, (all of whom have sung their roles on stage). Alain Lombard conducted. The recording was a miscalculation on all levels. The singers - even the remarkable Miss Freni - were in poor voice. Miss Caballe should never have been singing the role, and the conducting was inferior.

Herbert von Karajan has now decided that Turandot is really the province of lyric sopranos, and he has given the role to Katia Ricciarelli on his new recording (Deutsche Grammophon digital 2741 013). Miss Ricciarelli is in the throes of a dreadful vocal crisis, which this role only exacerbates. A once-attractive voice has in a mere five or six years become an unmanageable, wobbly, ill-tuned instrument. This follows from her insistence on singing roles far beyond her vocal means. That she never offered a strong artistic profile or sense of musical identity has been evident both on records and on the opera stage. That lack finds her at a particular disadvantage in the role of the imperious Princess Turandot.

It hardly helps that von Karajan draws every tempo out to near-stasis and generally lets the gorgeous Vienna Philharmonic overpower the sound stage. Mr. Domingo is the Calaf. In some pages, he is ringingly fine. In others, he is forcedly out of tune (no doubt in different form at different recording sessions). Barbara Hendricks is the fragile Liu, and she is overtaxed from beginning to end: Much of her singing is lovely, if somewhat monochromatic. Rarely does von Karajan allow her the sort of tempo that enables her to phrase naturally and expressively.

The orchestral contributions are unquestionably dazzling. This is the only recording of the work to get across the sense of oppressive weariness in the music, a sense that the barbarity of the Peking people is enforced and wearying. Everyone in the opera is oppressed by burdens, and von Karajan communicates this oppression stunningly well.

In the end, it is the most beautiful recording of the opera orchestrally, even if there are some surprising mistakes in the playing that indicate haste in the sessions. Also, in a shockingly poor tape splice, Miss Hendricks actually sings one-half of Turandot's phrase, while Miss Ricciarelli sings the other half! Nevertheless, vocally the set is so seriously compromised that even such lavish bit-casting as Siegmenund Nimsgern's Mandarin go for naught.

If the trends continue that two of the three above-mentioned recordings reflect, any sense of authenticity in opera will vanish from the recording scene - and eventually the stage - sooner than anyone would have imagined. A quick change back to saner practices is, with apologies to the Bard, a transformation devoutly to be wished.