Earlier quoted context omitted.
As I remember this was done on an Apple computer. In the command line you could do this with they 'say' command. The parameter -v is used for the voice. And I think "Fitter Happier" is 'Fred'. say -v Fred "fitter.happier.more productive" Edit: other sources say it was 'Bruce'. But I don't have an Apple, so I can't check.
My go to favourite is 'Karen' for the Australian accent. Her voice is far more natural sounding that most of the others I've auditioned, and I use her as the default and pepper all my deployment scripts with 'say' commands now.
She gets 'arvo' and 'stone the crows' right though, so that's something ;-)