He was told by the Apple PR to not publicly speak about Siri because Apple employees are not authorized to discuss products. But because Jon Briggs recorded his voice for a company called Scansoft, he never signed a contract with Apple, which is why he has now gone public.
Briggs recorded over 5’000 sentence over three weeks. He was told to speak in a specific way, reading “flat and even”. The recordings were then analyzed to grab specific phonetics. These phonetics now allow Siri to speak about every word in a natural language, even if Briggs never actually recorded that specific word.