It's hard to define and pin point what a "voice" is. Think about how you would describe the difference in voice of Tolstoy vs Dostoevsky, or William Gibson compared to Thomas Pynchon. You might have to read a lot of a writer to be able to notice their voice. You might even notice how a short story by an unknown writer resembles the voice of another.

Or think about reading something that might be stuff you could say in your own words but not actually in your own voice. Or maybe the opposite is a better example for you? That's what some poetry is about. The gap between voice and word.

Stylometry is the study and measurement of text, so that can go some distance to quantifying the difference between voices and can help to programmatically identify a human voice but I feel that because a voice comes from a person, the recognition is also mainly a human response.

At another level its about communication, person to person with the intention and desire to communicate something. LLM text output may seem uncanny as it's hard to emulate the intention and desire and empathy present in genuine human communication.

I would be interested in reading what poets say about LLM voices. I wonder if many LLM labs employ them?