The computer voice was an audio interface program designed to allow computers to express information verbally. Many civilizations, such as the United Federation of Planets and the Cardassian Union, equipped 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, as well as giggle. (TOS: "Tomorrow is Yesterday")
While most computers had female voices, the M-5 multitronic unit had a male voice (TOS: "The Ultimate Computer"), as did ISS Enterprise (NX-01) and the ISS Enterprise (1701). (ENT: "In a Mirror, Darkly"; TOS: "Mirror, Mirror")
Background information Edit
The computer voice on most Federation starship and fixed installation computers was portrayed by Majel Barrett-Roddenberry since the early days of the original series, and continued to play Starfleet computers on TNG (with the exception of some early episodes), DS9, VOY, and many Star Trek films through 2009. Star Trek was her final performance both as the computer voice and in a Star Trek project before her death.
In Star Trek III: The Search for Spock, the computer voice of the USS Excelsior turbolift was played by director Leonard Nimoy, under the pseudonym "Frank Force". Harve Bennett provided the flight recorder voice in the same movie, during playback of scenes showing Spock's death from The Wrath of Khan. Judi Durand did a 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 The Search for Spock.
The computer voice in TOS was very rhythmic and mechanical. In the later series, it became a far more normal-sounding female voice.