This is not likely to be a trademark issue (or a copyright issue for that matter).
What it might be is a personality rights (right of publicity) issue, which is governed differently in every U.S. state.
If someone impersonates Freeman’s voice or Hawking’s voice, it could give rise to a claim of misappropriation of one’s image, identity, likeness, or voice, depending on what the law of the particular state has chosen to protect.
Reasons why it can’t be a trademark issue – A trademark is a symbol, image, word (or possibly sound, color, or odor) that is used in commerce to identify of the source of goods and services.
A person’s name used just as a name is not considered such a use in commerce and I’m pretty sure that a person’s voice wouldn’t either.
For Hawking to claim his voice as a trademark, he can’t just say “this is my mark because it’s what I sound like when I talk.” He has to be offering goods and services and he has to show that he intentionally uses his voice as the brand for those goods or services. And even then, I can’t see how he could make a claim for anything and everything that might possibly be spoken using that voice. He would have to identify a particular word or phrase. And then he would also have to show that the general goods-or-services-purchasing public used that voice to identify what they wanted to buy.
Also significant is the fact that some other commercial entity made that voice for him, presumably for use by more than just one of its customers.
Someone would have to prove that it was Hawking that made the call. The fact that it is a lot easier to fake Hawking’s voice than it is to fake someone else’s voice would make it harder to prove.
My recommendation is to forget that the words “trademark” and “copyright” have the part participial verb forms “trademarked” and “copyrighted.”
The way that the law works, the question is almost never whether something has been “copyrighted” or “trademarked.”
Copyright protection attaches automatically upon creation of a creative, original work of expression that is fixed in a perceivable medium.
Trademark protection attaches based on the use of a distinctive word, image, etc., in commerce as the identifier of the source of goods or services. Someone might have “registered a trademark” (not “trademarked”), but that doesn’t necessarily mean that they actually hold exclusive rights.