Time and adversity have not been kind to Barbara Bonney, once the possessor of a silvery, clear soprano that responded to her every musical intention. After time off for personal reasons, she has been attempting to place her voice once again on its former secure plane. But the voice will not cooperate: It is unresponsive and unyielding. Gone is the pliability, the suavity, the subtlety — and with them, the artistry.