Newborn babies can hear fairly well, but not perfectly. The middle ear of a newborn is full of fluid and this impairs hearing to a small extent. Additionally, the entire hearing apparatus is somewhat immature. That's why newborn babies respond best to high-pitched, exaggerated sounds and voices.
Because they learn it in the womb, newborns are born with the ability to distinguish their mother's voice and respond to it above all others.
When to Worry About Your Baby's Hearing
If your newborn does not respond to loud sounds with a startle reaction or never seems to react to your voice in the first months, run it by your pediatrician. Most states now require newborn screening for hearing loss, so you will know early on that your baby's hearing is OK. However, newborn screens can miss some kinds of hearing loss -- so if you’re concerned about your baby’s hearing, bring it up with the doctor, even if the newborn screen was normal.