Quoting from https://web.archive.org/web/20181118114804/http://imsai.net/...

“Director John Badham states in the commentary that the actor voicing the raw content that was later modified for the computerized effect was John Wood (the Falken character), reading the script word-for-word in reverse order in order to portray a "flat quality" with limited inflection. That raw audio was then edited and re-assembled after being run through audio processing equipment to achieve the desired effect.”