Babies prefer human voices to other sounds and are especially drawn to the voices of their caregivers. After 1 month, they can identify primary caregivers by voice, even in another room. By the second or third month of life, babies begin smiling in response to the sound of familiar voices. They engage in “smile talk,” which can include gurgling or making other noises to attract attention and then beaming enthusiastically to their caregivers.
Shelov, S. P. (Editor-in-Chief). (2004). Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.