Who are you really talking to?
AT&T Labs will begin selling speech software it says is so good at reproducing sounds, inflections and intonations of human voice, that it can recreate voices and even bring those of long-dead celebrities back to life; Natural Voices software, which turns printed text into synthesized speech, makes it possible for company to use recording of person’s voice to utter things person never actually said; utterances still contain few robotic tones and unnatural inflections, leading competitors to question whether it is substantial step up from existing products; tests suggest it may be first text-to-speech software to raise specter of voice cloning, replicating person’s voice so perfectly that human ear cannot tell difference; photo (M)