Female voices are usually clearer and easier to hear than male because of the pitch.

It could be that the female voice is perceived as being a gentle suggestion, as opposed to an order. Since most pilots are still male, it allows the man to feel that he is still the one in control of the situation and is not being ordered around by the computer. The following is a c/p of an article about this very thing. It makes a lot of sense to me, though the findings tend to contradict themselves. See what you think: "Whatever the comparison, people were unhappy with an automated male voice telling them how to use a new pedestrian crossing at a downtown Santa Rosa intersection. The question is: Will they be happier with a woman giving those orders? City officials hope so after complaints prompted the switch at Fourth and D streets. Now when people arrive at the intersection, a woman's voice tells them when to wait and when the walk sign is on. She even throws the word "please" in there. "It doesn't make the hair on the back of your neck stand up as much as the male voice did," said Tim Hassler, who owns Timothy Patrick Jewelers at the intersection. City officials had no idea they'd be walking head-first into a debate over gender roles when they installed Santa Rosa's only pedestrian scramble at Fourth and D streets this summer. The system allows pedestrians to cross the street in any direction they please once all the lights turn red. Each time the lights turn green, the automated voice intones, "Please wait. Do not cross." No one has complained about the automated voices used at about 35 pedestrian crosswalks spread across town. Then again, all of those voices are female. The intersection of Fourth and D "is the only real complaint we've gotten about any of the voices that we use in Santa Rosa," said Rob Sprinkle, the supervising engineer in the city's traffic division. This situation shows what researchers have long known: When it comes to giving orders or directions, the gender of the person doing the asking really does matter. Blame it on your parents. Frank Farley, a psychologist at Temple University, said the fact people are complaining about the male voice may be because fathers -- meaning males -- generally are viewed as the disciplinarians in the family. In a wider societal context, he said males also are associated with control and power -- attributes some find unsettling in an age of increased surveillance and war. "The male voice lends itself more to that fear," said Farley, past president of the American Psychological Association. "If they get a more soothing female voice, it may be that people don't care so much about the rule aspect of it." In deciding to go with a female voice, Santa Rosa officials are erring on the side of comfort versus control, said Clifford Nass, a Stanford University communications professor and author of "Wired for Speech." "That really is the question of life in the 21st century," he said. "Where is the line between safety and comfort?" Such tensions long have fascinated Hollywood. It's no accident, for example, that director Stanley Kubrick chose a male voice for the evil computer HAL in the movie "2001: A Space Odyssey." The computer voice in the original "Star Trek," on the other hand, was female, ostensibly to show that Capt. James T. Kirk was in control, Nass said. "One of the biggest themes was, no matter how advanced the technology, it's humans that are in charge of the big things," he said. A key finding of Nass' research is that people are more apt to follow directions given to them by those of their own gender. As one example, he said engineers had to replace the female voices used for navigation systems in BMWs sold in Germany because German men refused to follow directions from a woman. But if the goal is to persuade a person of an unknown gender to do something -- say, to wait before crossing the street -- Nass' research suggests it's best to default to a male voice, as it is generally viewed as being more authoritative. Sometimes it helps to have a familiar voice. In the 1990s, Temple University installed an automated voice at a busy intersection using the words of Colin Powell, then an Army general. "The signal is green to cross Broad Street!" the general intoned at the signal, which since has been switched to beeping sounds. Some municipalities choose to record people who live locally to give voices an authentic feel, said Doug Gubbe, vice president of Novax Industries Corp., a Canadian-based company that supplied Santa Rosa's voices. Santa Rosa's decision bucks these trends. Sprinkle said the decision was made to go with a female voice after people complained the male version was too "harsh." "We don't want to annoy people," he said."

I have no preference for the voice in the cockpit. Some may be perceived as easier to understand but I don't see it as an issue. Regardless of the implied gender of the voice, it will be designed in such a way that the pilot is able to easily and instantly understand it. The jet that I fly, a Canadair CL-600 CRJ 200, uses a male voice for the cockpit alarms and instructions. I recently rode in the cockpit of an A330 in the jumpseat and noted that it's approach numbers were spoken by a male computer voice. The old cargo DC-8 that I flew as a first officer had no computer voices, only alarm tones that we were trained to recognize. I do not know where you got this information about a female computer voice. The 747 uses a male computer voice. Watch a few cockpit videos of the 747 on finals and you'll hear the computer.

Maybe they think a womens voice is more soothing?

I fly on a variation of a Boeing aircraft that uses the following message in a female voice: "Altitude" (when 1,000 feet above or below the desired altitude) And the following messages in a male voice: "Terrain, terrain" "Pull up, pull up" "Caution" I would have to agree that the soothing female tone is for those calls that are reminders or things to take note of but not directly focus your attention on... conversely, the male voice is more directive in nature, requiring some form of immediate actions on the part of the pilot. It wouldn't surprise me if Boeing has done extensive research on the reaction of pilots to each of the audible calls, and chose the voice that worked best based on the results of the studies.

not sure why, maybe they just happened to have a girl do it

It is just the default setting and no one ever bothered to change it.

Probably because most of the engineers were guys... Possibly (I'm guessing here) because engineers and /or pilots refer to their craft as "she", just as do shipboard personnel.