The elusive magic of a Broadway musical

Original-cast albums of Broadway shows have always had their problems. It's not possible to capture all the excitement and immediacy of a live performance on record, and some show recordings seem downright lackluster, compared with the real thing.

Today performers go to a recording studio and the various elements in the musical production are recorded separately, one by one. Then the singers superimpose their voices over the instrumental tracks. Sometimes a certain passage will be reworked again and again until it's ''perfect.'' But is this so-called perfection what is most important in a show album?

The first to disagree would be Maury Yeston, composer of the musical score for the award-winning Broadway show ''Nine,'' and RCA recording engineer Mike Berniker, who, along with Mr. Yeston, came up with the idea of doing a live recording of ''Nine.''

What exactly is a live-show album? Yeston explained that the recording was actually done in the studio, where the entire show was performed, just as if it were a theater, then edited afterward. The point of this approach was to capture the spontaneity of a live performance.

Why didn't they just take a recording crew into the theater and record the show in progress, with the audience sitting right there, clapping, laughing, and so on? Yeston explained that such an approach would lose a lot in sound quality.

So Mr. Yeston and Mr. Berniker set up the show in the studio, where the actors and actresses could assume their roles and feel the continuity of a complete performance as it was being recorded. Yeston explained that this way the actors were able to maintain ''the tension and excitement of the show, and of course in theater music the whole impulse for the song has to do with what's come immediately before it. So we recorded - even though we didn't put it on the record - the dialogue, and everything that led up to the song.''

Yeston also pointed out that ''Nine,'' perhaps more than some other shows, is well suited to this type of recording.

''It's a highly integrated show. That means that the plot, the story, and the important characterizations happen in music and lyrics.The songs in this show are not like raisins stuck in the middle of a cake - they are the show.''

But did this live-recording process really make all that much difference from the listener's standpoint? Some critics of the album have pointed out that what it gained in spontaneity, it lost in quality to some extent, citing specifically the performances of Karen Akers and Raul Julia, neither of whom seemed to be in particularly good voice. Yeston disagreed.

''When you do a Broadway show, you come to understand that you're working with singing actors and actresses. Some will sing better than others, and some will act better than others. The important element in musical theater is not always absolute perfection in singing, but rather, feeling.'' He added that some people simply don't like the way Miss Akers and Mr. Julia sing.

Both Yeston and Berniker seemed satified with the results of the album. And although some might argue about the quality of the singing or other aspects of the recording, it's clear that both approached the project not only with much enthusiasm, but with a good deal of integrity and skill.

The long-playing album consists of a single record containing some 68 minutes of music. RCA has also issued a cassette containing more than 80 minutes of music from the show.