Re: sex sells
I've heard different, reasonable-sounding justifications for assistants, and earlier even, voice navigation, using female voices.
As human we're more acutely tuned to female voices... possibly due to our early, formative years spent around mom. Secondly, noisy environments, e.g. cars, tend to drown out lower frequencies, given higher pitched female voices a slight advantage. And thirdly, people tend to be more compliant accepting instructions from women. The latter could again be a side effect of our early years spent with mothers or possibly a way to bypass the male competitive streak.
I'd love to hear from anyone with more official knowledge though.