Newborn babies hear fairly well, but their middle ears are initially full of fluid which muffles things for a while and their internal hearing organs are not fully developed until about 6 months old. It takes them a while to make sense of their auditory world.
The one thing they are born knowing very well of course is the voice of their mother, which they have been listening to from inside the womb (albeit through a layer of tummy and amniotic fluid!) since about 18 weeks prenatal. As their hearing clarifies through the early months of life, the most wonderful thing for a baby to hear is usually the voice of his/her parents, because that particular noise is associated with feeling safe, secure and having all their needs met.
During our baby massage classes we start to introduce a few songs to accompany our massage routine, nothing too taxing, a quick burst of 'Twinkle Twinkle' here, maybe some 'Row, Row, Row your Boat' there the next week. I've noticed that a new group of mummies are usually rather reticent to start singing the first time it is suggested to them, but just seeing how their babies react to their voices while they have eye-to-eye contact during massage is enough to banish all future shyness away. Babies LOVE hearing their parents sing to them!
Have a look at this little beauty listening to her Mother sing to her, she's absolutely adorable.