Adults usually hear in a narrow band of sound, while babies seem to use a different approach. They don't have the selective attention of adults and they don't pay attention all of the time. Instead they always seem to be listening broadband or to all frequencies simultaneously.
In today's western culture a baby is at a great disadvantage. All the noise we expose people to makes it difficult for babies. The practical lesson from this research is, if you are talking to a baby or reading her a story, background noise can be a problem. Turn off the television or radio. (EurekAlert, but don't bother because I've pasted all the good parts)