The sound of Hawkings' "voice" is a DEC Talk, a text to speech generator that has been around for over thirty years. Digital Equipment Corporation originally developed this device for automated phone responses ("Press one for accounting, press two for sales,.."). Because of the high quality of the speech synthesis, it became popular with blind users of computers, including a few that I helped train to use computers WAY long ago. For a sample of this speech, check out
http://www.speechfxinc.com/dectalk.html
Note that the DEC Talk can be set to "speak" as Perfect Paul, Beautiful Betty, Huge Harry, Frail Frank, Rough Rita, Kit the Kid, and others.
I recognized Dr Hawkings as using "Paul" as soon as I heard his synthesized speech, many years ago.
https://en.wikipedia.org/wiki/DECtalk#In_technology_and_culture
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Mr Smith: Like the rose trampled on the ground, you took the fall

The "voice" of Stephen Hawking is actually antiquated technology. Because he is so iconic many members of the technological world have kept it up to date with current verbiage and syntax.
The man himself has spoken of it through said "voice" several times through the years that he is so comfortable with it that while newer tech would give him a less robotic "voice" that is what he is known for and has no plans to change it in his lifetime.

Dr. Hawking's is an early computer-generated voice. Newer ones can be argued to be "better" in various ways, but since the one he's using is now iconically associated with him, probably a better idea to stick with it.
Howard is an engineer. He might have had an old copy of it lying around and just had to port it to his own usage.
http://www.bobpeters61.com/linkables/CaligulusTrumpus.gif

Im sure you can download a program or something.
Good luck in finding one. If you do, please let me know.

Im sure you can download a program or something.
Lose the Game!!!!!!!

Howard does a Hawking impersonating, starting even in the very first episode. Maybe he did it himself, with enough retries until it sounded good enough. And with that electronic reproduction it didn't really have to be exact anyway.