For over 40 years (out-of-universe), actress Majel Barrett provided the default or standard computer voice aboard most Starfleet starships, including the original Enterprise, the Enterprise-D, the Defiant, and Voyager. Ms. Barrett also portrayed several human or humanoid characters at the same time, including Una Chin-Riley, Christine Chapel, and Lwaxana Troi.
From an in-universe perspective, at any level of canon, has it ever been addressed where the voice of the computer comes from or who might have provided it? Was the voice based on a specific Starfleet officer who worked on their talking computer interface development? Were Chin-Riley or Chapel recalled to Earth or Utopia Planitia to record their voice pattern for the synthesizer? Is the voice supposed to be a fully artificial voice driven by Artificial Intelligence (AI) research on what Starfleet officers find most pleasing?
Chin-Riley and Chapel seem like plausible options. Chin-Riley became famous as an officer in the ten years leading up to Kirk's command, and Chapel could have been selected as a result of her clinical training and a desire to portray the computer as displaying empathy. If Betazoids have similar lifespans to humans, then it seems unlikely that Lwaxana Troi was the source as she would likely have not been alive during Kirk's first command of the Enterprise. The opposite is also possible, that Chin-Riley, Chapel, and Lwaxana Troi modeled their own speech on that of the computer, possibly as part of some accent-reduction training or speech therapy. These are, however, just speculation.
The most likely in-universe options seem to be:
- The voice was provided by or based on recordings of a character who was originally portrayed by Majel Barrett, such as Una Chin-Riley or Christine Chapel.
- The voice was provided by or based on recordings of a character who would most likely have been portrayed by Majel Barrett had she ever appeared on-screen. This would cover, for example, an "unnamed programmer" or "admiral's favorite granddaughter" situation.
- The voice is an AI construct trained with hundreds or even thousands of sample voices and adapted to what Starfleet officers reported is most pleasant coming from a computer. In other words, it's not based on any specific person but rather a specific personality, archetype, or background. The fact that the voice is practically indistinguishable from those of several historical Starfleet officers and Troi's mom is either coincidental or based on those characters imitating the computer, either unconsciously or as part of a speech therapy or accent reduction program.