Here are the answers from WikiAnswers:
The answer may have to do more with the psychological fact that women need to be more alert to their senses when taking care of an infant, than the actual physiological difference. They have the same hearing apparatus as men, but they focus on it more during child rearing.
The other factor that can be considered is that the corpus callosum of women are 23 percent larger than in men. So that means that the left and right hemispheres communicate more effectively, and this can be a factor in interpreting the sounds babies make, because baby sounds are processed in the right hemisphere.
So, maybe it is not so much that they can physically hear better, but that their brains are designed by nature to pay particular attention to selected sounds, and that may be considred that they can hear better than men.