'Hallelujah! Hallelujah!': sing-along choruses tune up for season's joyous 'Messiah'

'' 'All we like sheep . . . ,' '' a burly lawyer in a three-piece business suit intones in perfect paradox - and not-so-perfect pitch. But no one else in the living room notices, as members of the neighborhood chorus flip the pages of the oratorio score and chide themselves for singing ''a monotono.''

Across town, on the top floor of the 48-story Transamerica pyramid, a motley corporate chorus of accountants, maintenance workers, and underwriters sacrifices lunch breaks to practice its singing.

These shower-stall virtuosos, from the jumble of Chinatown to exclusive Presidio Heights and the quiet suburbs of the East Bay, are priming themselves for the big-time here: George Frederick Handel's masterwork, ''Messiah.'' Or rather, the San Francisco Conservatory of Music's version of it - the ''Sing-It-Yourself Messiah.''

Musical perfectionists are likely to cringe at the thought of some 3,000 San Franciscans packing the elegant Davies Symphony Hall tonight and tomorrow night (Dec. 5 and 6) to have the conductor turn and lead the audience through what, even to the educated ear, is a Rubik's cube for singers, an undertaking normally preserved for the ''serious'' singer.

But that's just the spirit of the conservatory's holiday season fund-raiser, say organizers and past participants, who reminisce about audience-participants who won't stop singing even after the conservatory's student orchestra has packed up and left.

''People are starved to sing,'' says Milton Salkind, conservatory president, accounting for the tradition ''the people's 'Messiah' '' has become in the short course of five years. Why else would people pay as much as $35 a seat to sing en masse, and not want to quit? he asks.

''It's more fun with your mouth open and not just your ears,'' offers Martha Sternberg, a literary agent who sings alto with a Presidio Heights neighborhood chorus.

Louis Magor, who conducts the 3,000 participants and the conservatory's orchestra, has a strong defense for those who speculate about the difficulty of bringing the disparate and unrehearsed voices of a city together in a mass ''Messiah.''

''I never feel like I'm in control,'' he admits with a laugh, ''but it's always worked. It's a very hard piece, but singing is something everybody has done at one point. And it's not like a dance-it-yourself 'Nutcracker' suite - you're not going to make a fool of yourself.''

''This really isn't a performance, it's a get-together. If you make a mistake , there's no penalty, no critic to say it's bad,'' concludes Mr. Magor, who ignores the picky subtleties and focuses on generalities in an extension course called ''Learn it with Lou.'' This year 150 ''Messiah'' participants took the class at the conservatory, which is violinist Isaac Stern's alma mater.

This community-type ''Messiah'' is not unprecedented, says Laurette Goldberg, a faculty member at the conservatory. In Sydenham, England, an 1859 performance at the Crystal Palace drew 2,765 participants.

Funded by Chevron and broadcast locally at Transamerica Corporation's expense , the ''Sing-It-Yourself Messiah'' earned the conservatory $80,000 in two sellout performances last year. This year, Transamerica will fund the national broadcast of ''Messiah'' on PBS stations.

This is not the only community ''Messiah'' done in the country. At the Kennedy Center in Washington, D.C., a professional choral group with guest soloists leads the audience. The event has been held since 1971, and this year's took place on Saturday. Boston, New York, and Pittsburgh have well-known sing-alongs, and Chicago has one annually in Orchestra Hall. But San Franciscans like to think they have perfected the idea.

''This seems more San Francisco than anything else,'' Mr. Salkindsays. For example, the Chicago ''Messiah,'' in his view, is ''too serious'' - it includes choral groups that rehearse all year. The atmosphere, he says, doesn't invite the unrehearsed to join in and have a good time. San Francisco's sing-along, on the other hand, is stereotypically Californian in its informality.

Magor says a large part of the fun of conducting this event is that, turned to the audience, he can see individual faces registering every goof and every success. And most of all, he says, letting the layman belt out ''Messiah'' ''makes people feel good.''