What Your Baby Should Hear
Although sounds are muffled by the surrounding amniotic fluid, your baby can hear even in the womb -- and she'll probably come to know your voice quite well. Her hearing gets sharper over the first year of life, and she also learns to recognize where sounds are coming from. For the first three months, your baby will only turn toward a sound that comes from in front of her, but by 6 to 12 months she should also be able to track sounds coming from behind her or from the other side of the room.
Your infant prefers high-frequency tones like the female voice, probably because she can hear them better. Maybe that's why it's natural for adults to adopt a high-pitched singsong voice (also known as "motherese") when talking to babies. By 4 to 6 months your baby should start to repeat recognizable sounds and babble.