Earlier quoted context omitted.
It’s definitely sketchy (classic OpenAI) But my question is: is what they did actually illegal? Can someone copyright their own voice?
Yes, absolutely illegal. You don't need to copyright anything, you simply own the rights your own likeness -- your visual appearance and your voice. A company can't take a photo from your Facebook and plaster it across an advertisement for their product without you giving them the rights to do that. And if you're a known public figure, this includes lookalikes and soundalikes as well. You can't hire a ScarJo imperson…
Just find someone who sounds like her, then hire them for the rights to their voice.