Seems that it’s hard:
“There is no such thing as a voice print,” he said. “It’s a very very dangerous term. There is no single feature of a voice that is indelible that works like a fingerprint does.”
Many different factors influence how people speak at any particular time and place.
“If you’re tired or if you have a cold or if you’re speaking on a phone against traffic in the background you do all sorts of things to the voice, which make it phonetically very different from time to time,” said Foukles, who also works as a freelance consultant for a private forensic speech science laboratory.
“The features of speech and language are such that you can’t use them as a marker of identity to identify one person and exclude all other people under normal circumstances. People’s voices overlap.”