I was walking through a high school a few years ago, (for a legitimate purpose), when I overheard a computerized voice speaking from a classroom.
I recognized it as the voice of Stephen Hawking. Sure, anyone could have his words read by a computer and sound the same.
But people will only listen to Stephen Hawking doing so.
There was TV show on Britain's Channel 4 called 'TV Offal' - as the name suggests, it showed weird stuff that didn't make it with the censors or the editors.
Anyway, the start of the show would always feature Stephen Hawking - and he was quite funny, at one point he got his voice machine to laugh:
"Harharhar, hardyharhar"