There’s a lot of research going on. It is evident that the unborn baby is able to listen to music quite early (21 weeks if I remember correctly). But most likely the influence of music is more important during the first 2 years after birth. Why? I might have to do with the phenomenon of
http://en.wikipedia.org/wiki/Synaptic_pruning
“In neuroscience, synaptic pruning is a neurological regulatory process, which facilitates a productive change in neural structure by reducing the overall number of overproduced or “weak” neurons into more efficient synaptic configurations. The purpose of synaptic pruning is a simple means of removing unnecessary neuronal structures from the brain; as the human develops, the need to understand more complex structures becomes much more pertinent, and simpler associations formed at childhood are thought to be removed for more complex structures.”
There is also a lot of evidence that listening to music (which has a healthy minimum of melody and complexity) as well as playing a musical instrument increases general intelligence, emotional and social intelligence. Children become far less violent.