Jane Powell will have the Nellie Forbush role opposite Howard Keel when "South Pacific" opens on Wolf Trap's bigstage Monday night. That's the night the original Nellie, Mary Martin, opens at the Eisenhower in "Do You Turn Somersaults?" In her new role Mary hums a waltz but doesn't sing.

The Wolf Trap run is only for a week, with two performances the final day, Saturday, matineee at 2, evening at 8. Donald Driver's cast, which has been touring Texas since spring, includes some recognizable names, Brandon Maggart (as Luther Bills), Igors Gavon, Casey Walters, with Queen Yahna billed as Bloody Mary, James Ferrier as Cable and Joanna Pang as Liat.

Starting Tuesday and nightly through Saturday at 6, Wolf Trap will have "preperformance preps" on musical theater in its Meadow Tent. These will be free but reservations should be made at 281-5587. That's the site for Interplay Productions' "Happy Landings" performances mornings at 10 and 11.

The new two-record album of the Scottish Millitary Tattoo's Bicentennial visit to Wolf Trap, a smashing array of sounds recalling the Duke of Edinburgh's visit, is one sale in the WOlf Trap gift shop.

"This summer's joint auditions by Arena Stage and the Folger Theater Group for minor roles, walk-ons and modest assignments during the coming season will be by appointment only at 554-9066. Audition dates will be Aug. 29 though Sept. 2. Leaders of both companies will be in the Old Vat Room at Arena to save time.

A Gotham hit of 1906 was "The New York Idea" by Langdon Mitchell starring the famous Mrs. Fiske. It was revived last spring and again admired.Now his daughter, 81-year-old Valentine Mitchell Gammell, once a protege of Stephen Vincent Benet, has a play about Ferdinand, Isabella and the intrigues of that 1492 voyage, "Make New Banners." It's playing through this Saturday and again next Wednesday through Saturday at 9 p.m. at Georgetown's Grace Church, 1041 Wisconsin Ave., NW.

Olney Theater Residence has two disparate casts. Continuing through Sunday is Chayefsky's "The Tenth Man," set in a synagogue with such noted Jewish actors as Albert M. Ottenheimer, Morris Engel and Bernie Passeltiner. Rehearsing to open Tuesday in Friel's "Philadelphia, Here We Come," about an Irishman's voyage to America, are such recognized Irish players as Pauline Flanagan, Jarlath Conroy, who was in Broadway's "Comedians," Helena Carroll (playwright Paul Vincent's actress-daughter) and Tom-Patrick Dineen. "It's gorgeous," proclaims Helena.

Carter Barron next week greets the return of the all-black company of "Guys and Dolls," which played the National before Broadway. Leslie Uggams is Sarah Brown this time around, with Richard Roundtree, Deborah Allen, Maurice Hines and James Spinks in other leads. It plays Tuesday through Sunday at 8:30 p.m.

Craig Babcock's "Mime for Everyone" is this noon's attraction in the Streets for People series in Gallery Place Mall, F Street between 7th and 9th streets. Through the end of the month Babcock also conducts his workshops at Glen Echo Park.

The Honolulu Theater for Youth will perform its "Tales of the Pacific" Sept. 9 and 10 in the Children's Arts series at the Kennedy Center. Wallace Chappell's group is on a mainland tour with this offering which so far has played to nearly 45,000 people on the five Hawaiian islands.

Jule Harris will play her Tony-winning "The Belle of Amherst" at the Edinburgh Festival early next month, after which she moves to London's Phoenix Theater, opening Sept. 14 . . . "Belle's" director, Charles Nelson Reilly, is staging James Earl Jones as "Paul Robeson," which will be at the National in December . . . Carol Channing's "Hello, Dolly!," a sellout in every city she's played, is now the closest she'll be for a year, Wilmington, Del., through Saturday . . . Anita Loos, whose "Gigi" is next for the Little Theater of Alexandria, has just gotten an advance check for the Tokyo production of "Lorelei," where, says Loos, "its heroine will be called Rorari Ree." Hugh Martin is already writing a song about it . . . Look for Hermione Gingold to join the New York cast of "Side by Side by Sondheim" come October . . . Josef Sommer, who went from "The Shadow Box" to the Trapier's hower's "The Archbishop's Ceiling" to the Trapier's "Voice of Pride," has rejoined "The Shadow Box," but now he has Laurence Luckinbill's major role.