THE CASE of the Missing Wallflowers: Several rolls of really tacky wallpaper -- intended for the set of the campy murder mystery "Shear Madness" -- vanished last week only to turn up on the bathroom walls at a Washington hotel.

"{Co-producer} Marilyn Abrams and I always pick out the wallpaper for each new 'salon,' " said Bruce Jordan, the show's director and co-producer. "I didn't want it delivered to the Kennedy Center for fear it would get lost somewhere in the vastness, so I had it sent to me at Guest Quarters. Well, the hotel was also expecting wallpaper, and the workmen must not have looked at it very closely, because they papered two or three bathrooms."

Jordan described his dramatic wallpaper as "yellow with white and pink Shanghai lilies," and said Guest Quarters management has been "absolutely fabulous" about replacing it. But Jordan says the hotel declined to leave the wallpaper up in their bathrooms: "Beauty is in the eye of the beholder and all that."

"Shear Madness," which begins previews August 12 at the Kennedy Center Theater Lab, involves the audience as armchair detectives who help solve the murder of a concert pianist found stabbed in her apartment, above the Shear Madness unisex hair salon. The Kennedy Center's turning the Theater Lab into a 350-seat cabaret-style theater, with some floor-level tables and chairs; so patrons will be able to bring in drinks.

While performing in a LORT theater in upstate New York, Jordan and Abrams discovered a 20-year-old German script called "Scherenschnitt" ("Cutouts"). They obtained American rights, adapted and updated it, and presented it in Boston eight years ago -- and the show is still selling out. With "franchises" in Chicago, Montreal and Barcelona, the semi-improvisational show changes daily to accommodate local names and events. The producers held open auditions and cast three Washington performers: Robin Baxter, who plays a tart-of-gold manicurist, Michael Gabel as a seedy antiques dealer from Langley Park, and Steve O'Connor, a junior at American University, as a preppy undercover cop.

Even though there are five performers to share the vocal duties, the 22 songs in "A Sondheim Evening" still place an extraordinary strain on a singer's voice. "Just in terms of learning the music, Stephen Sondheim is one of the most difficult composers to sing," says Brian Davis, who has performed work by composers Kurt Weill, Harold Arlen and Jacques Brel in previous D.C. Cabaret songwriter revues. "His songs are very complex melodically and rhythmically, and he chooses difficult, very chromatic melodies. So there's no room for error, especially if you're singing with someone else."

"And the songs are all over the place in terms of range," says Ann Johnson, who sings a thrilling "Not a Day Goes By," one of the evening's highlights. "Sometimes I say, 'Okay, Mr. Sondheim, this is great, but when do I breathe?' "

Because the show is so vocally demanding, Davis -- like many singers -- shuns dairy products to avoid "yukking up" his voice, and prefers to stay as silent as possible during the day, though that's more difficult lately, as he teaches young people's acting classes at Round House Theater. Johnson, who's on the phone for hours at her day job, adds that a singer's lot is not -- to quote another Sondheim song -- "The Glamorous Life" it seems. "Often we'd like to go out and have fun after the show, but if you want the voice to be there tomorrow, you have to just lie down and shut up."

"A Sondheim Evening," which packs in 30 years of Sondheim's best and least-known songs, stressing the personal side of the composer, was originally presented in 1983 at Sotheby's auction hall. The Washington production, directed by Peter Frisch with vocal arrangements by Roy Barber, has been extended at New Playwrights' Theater through August 16.

Bulletin Board: Gross National Product -- which has been doing such brisk business with its "Man Without a Contra" satirical revue that double performances have been added at d.c. space Friday and Saturday nights -- opens a version of the show in Los Angeles Saturday. Washington's Susan Baronoff directs again . . . Playwright/actor Mark Harelik, whose "The Immigrant: A Hamilton County Album" is winning hearts at Arena's Kreeger Theater, will speak Sunday at 2 at B'nai B'rith International Center, 1640 Rhode Island Ave. NW. It's free, but reservations are recommended. Call 857-6583 . . . Touchstone Theater's "Four Men From Annapolis" has been extended through August 30 . . .

Round House Theater has announced its full fall season, another impressively progressive roster of plays and playwrights, with an equally fine lineup of directors: Keith Reddin's "Rum and Coke" (directed by Susann Brinkley, begins October 8); Harry Kondoleon's "The Fairy Garden" and "Self Torture and Strenuous Exercise" (Max Mayer, December 3); George Walker's "Zastrozzi: The Master of Discipline" (Jerry Whiddon, January 28); Ibsen's "A Doll's House" (Jim Nicola, March 24); and Eric Overmeyer's "On the Verge, or The Geography of Yearning" (Gillian Drake, May 19). Call 468-4234 to subscribe . . .

Wildwood Summer Theater, the area's premier young people's theater, presents "Grease" for three weekends at Walt Whitman High School. Call 229-2160. And Rockville's Youth Theater is presenting the proto-yuppie musical "How to Succeed in Business Without Really Trying" at F. Scott Fitzgerald Theater. Call 424-8000 . . .

Washington Times theater critic Hap Erstein was one of the 15 brave playwrights who entered Source Theater Festival's 10-minute play competition. Erstein's play, "Confessions Tonight!," an amusing satirical amalgam of one-person shows, was quite well received at its "world premiere" Monday night. Who says those who can't do, preach? The winners of the competition will be announced at the festival awards night, August 11 at the Georgetown Omni Hotel.