Memory! The image above is sort of like our memory. Who knows what connects those little synapses in our brain to each other? Somehow it results in a familiar path, individual to each person, and that becomes: memory.
Ability Development From Age Zero by Suzuki p.8-9 mentions that babies will recognize the music the parent plays or sings to them repetitively. At about five months old, the child who has been sung to will smile or react when a familiar song is played. The same child will have a flat expression when a new song is introduced. Makes sense!
So, what are your little ones listening to?