A RYEH NUSSBAUM COHEN is just twenty-four, but he's already a countertenor on the rise. On Kenneth Fuchs's Poems of Life, released on CD in August, he sounds remarkably secure, gracefully traversing a tessitura that can cause some in his fach to strain.  "Most countertenors are struggling baritones [who] realize, 'Oh, I have a pretty nice falsetto—let me give this a whirl,''' he says, chuckling. "I'm the rare case that I've always been a countertenor. But not by any plan—by luck and ignorance. I was in children's choir, and my voice dropped. But I liked singing in the choir, so I kind of finagled and managed to keep singing alto. I always joke that my range dropped a minor third when my voice dropped.”