When she's singing at full power, her voice can nearly tear the roof off a theater. But on this particular morning, her voice is barely audible, faint and husky over the telephone line. Stephanie Mills has awakened with a sore throat.

"I'll have to miss my last day of rehearsal," says the pop singer, a couple of days prior to launching a two-month, 30-city tour that includes a performance at Constitution Hall tomorrow night.

But Mills isn't fretting. Despite the rude awakening, she's confident she'll be in good voice come showtime, and she's eager to go out on the road once again. "It's my life," she says. "I really think it's in my blood to perform. I'm at my best when I'm on stage. That's where I'm happiest."

The tour marks the first time she won't be performing any material from "The Wiz," the Broadway musical that won her acclaim when she was just 16. With seven solo albums and numerous hits and awards to her credit, she can well afford to focus on her own recordings now, but she's quick to point out that she isn't trying to disassociate herself from the land of Oz and Dorothy, the role of a lifetime.

"People still see me as Dorothy and I don't ever want to lose that association," she insists. "In my mind, there are only two Dorothys -- Judy Garland and me. That's great company . . . Early on I think it was more difficult for the public to separate Stephanie Mills the recording artist from Stephanie as Dorothy, but that was because I was so young. I've matured as a recording artist and I think the perception of me has matured, too."

If ever there was a time when Mills wasn't singing, she can't recall it. At age 3 she was entertaining family and friends at her home in Brooklyn. Six years later she landed her first Broadway role in "Maggie Flynn," starring Shirley Jones and Jack Cassidy. Then, it was uptown to the Apollo Theatre, where she won the "amateur night" contest for six weeks running and subsequently opened for the Isley Brothers. It was her first professional booking. She was 11 years old.

"I guess I was too young to be nervous back then," Mills says of her debut. "I know those audiences at the Apollo were really tough -- if they didn't like you, they'd boo and pull you right off. But I never even worried about it. I just wanted to sing. They seemed to enjoy me so I kept going back."

Despite the fact that Mills had already cut her first album when "The Wiz" beckoned, she gladly put her solo recording career on hold and headed for Broadway. Hardly a risky move, it would seem now, but Mills says the success of the show was far from assured. "People forget it wasn't a big success in the beginning. I think if it weren't for churches and community groups supporting us by the busload, we might not have made it."

Disappointment came years later, Mills admits, when the role of Dorothy in the film version of "The Wiz" went to Diana Ross. Just the same, Mills says she understood "the politics involved, the need for a box office name." As it turned out, she soon discovered the film was something of a disappointment, too.

"I think the story line changed too much and that's why it wasn't as successful as it could have been," she explains. "There was 'The Wizard of Oz' and then there was 'The Wiz' -- an all-black version of that. When you change the plot and characters -- Dorothy became older, a schoolteacher, and her quest was different -- when you start to change something that's already perfect, that's when you run into trouble."

Mills returned to "The Wiz" briefly two years ago, and recently appeared on a daytime soap. Acting, she says, is great fun and she still harbors the hope of making a name for herself in Hollywood. Until then, though, videos will do. "They're really mini-movies," she says. "You only have X amount of time to tell your story and I think my acting background really helps me to get the message across."

Videos, of course, are usually tied to hot singles, and Mills has had her share, including a couple of successful duets with Teddy Pendergrass. And she intends to keep them coming. Over the years, she's taken more and more control of her recordings. On her latest self-titled album, she oversaw the work of five producers, including such industry heavyweights as Rod Temperton and George Duke, making sure that each was attuned to what the others were doing.

Mills says she also pays particular attention to the lyrics she sings, though she doesn't side with those who want to see records rated for offensive language.

"I think it's important that each artist really believe in what they are singing about. I won't sing cheap lyrics that suggest drugs. I think we have a responsibility to the people who buy our records and attend our shows . . . if we'd keep in mind that children are listening, we wouldn't even be discussing the need to rate records."