Unless that soprano is Sonya Yoncheva, the brilliant 35-year-old Bulgarian whose silky, flexible voice effortlessly shifts from Handel’s quicksilver Baroque runs to Tchaikovsky’s Romantic heft.

“I believe that the human voice is unlimited,” she said by telephone from Switzerland, where she was home before leaving for New York to rehearse “La Traviata,” which opens on Feb. 24 at the Metropolitan Opera. “If I feel that I’m sensible and sensitive about the music, why not? If the voice is elastic enough, it’s O.K. to sing whatever your soul says.”