Matthew Aylett, CereProc's chief technical officer, said the company started working on the voice Ebert has affectionately called "Roger, Jr." soon after receiving his request.
"We typically use synthesis in all sorts of environments, but this is the first time we've used it in a way that really helps someone who needs it in this way," he said.
Usually, the company's text-to-speech technology is used by companies that need a synthesized voice for call centers or in-car communication, he said. Academics also work with CereProc on voices for more human-sounding, cutting-edge robots. But Aylett said that as the technology advances, the applications are endless.
In all cases, Aylett said, CereProc engineers and speech experts work to "infuse character in our voices."
In addition to creating regional-sounding voices, the company produces voices with variation and emotions. Though CereProc can't yet recreate the full range of emotions,it's possible to construct voices that sound more snippy, cheerful or sad, he said.
The work on "Roger, Jr." builds on efforts to clone speech.
"When you clone a voice, what you're trying to do is represent a character and get that personality out," he said. Using recorded audio of George W. Bush, for example, the company recreated the voice of the former president.
The prototype voice that Ebert demonstrated for Winfrey was constructed from about three hours of the critic's commentary on movies such as "Citizen Kane" and "Valley of the Dolls."
After transcribing the commentary, Aylett said the company broke down the words into the roughly 45 speech sounds that comprise the English language. Those sounds are also affected by Ebert's intonation patterns and the ways he expresses feeling in a sentence, he added.
The task was made more difficult because Ebert's commentary was recorded over different years, in different audio environments and with different equipment. Aylett said the transcription process was complicated by the fact that Ebert frequently spoke off the cuff.
Still, he said the company continues to add data to build Ebert's voice and expects to have a completed voice in the next few months.
"The voice communicates an awful lot about you," he said. "Your history, where you're from, what sort of person you are in many ways. It's also part of our soul. To create that, for example, for people who have lost it is a really exciting thing to do."
ABC News' Lauren Cox and Emily Friedman contributed to this story.