The computer voice is an audio interface program designed to allow computers to express information verbally. Many civilizations, such as the Federation and the Cardassian Union, equip their computers with this feature.
The computer voice of the USS Enterprise was reprogrammed in 2267 on Cygnet XIV, with the intent on giving it a less mechanized personality. The resulting modifications caused the computer to address James Kirk in an increasingly amorous manner. (TOS: "Tomorrow is Yesterday")
During reorganizations of Deep Space 9's computer in 2369, Chief Miles O'Brien experienced diffculties because of the previously Cardassian programmed computer systems. The female computer voice told him to read the Cardassian operational guidelines and refused to follow his commands. When Commander Benjamin Sisko reminded O'Brien that this is "only" a computer, O'Brien answered "This is no computer. This is my arch enemy." (DS9: "The Forsaken")
Background
The computer voice on most Federation starship and fixed installation computers has been portrayed by Majel Barrett Roddenberry since the early days of the original series (with the exception of some early TNG episodes). The computer voice in TOS was very rhythmic and mechanical. In the later series it became a far more normal-sounding female voice.
The computer voice on the ISS Enterprise in the mirror universe was provided by John Winston, the actor who portrayed transporter chief Kyle. (TOS: "Mirror, Mirror")
Doug Hale voiced the upgraded USS Enterprise computer alerts in Star Trek: The Motion Picture.
Marcy Vosburgh provided a computer voice in Star Trek II: The Wrath of Khan.
In Star Trek III: The Search for Spock, the computer voice of the USS Excelsior turbolift was played by Frank Force (a pseudonym assumed by Leonard Nimoy). Harve Bennett provided the flight recorder voice in the same movie, during the playback of the scenes showing Spock's death from Star Trek II: The Wrath of Khan. Judi Durand did the Spacedock computer voice that says "Danger... space doors are closed," while the USS Enterprise is escaping from the station. Teresa E. Victor was the computer voice for the USS Enterprise self-destruct sequence in Star Trek III: The Search for Spock.
The Cardassian computer system, as used on Star Trek: Deep Space Nine, was voiced by Judi Durand (who also voiced the Federation system in many of Activision's video game releases). Majel Barrett Roddenberry continued to play Starfleet computers in DS9 and Voyager.