Language acquisition starts at birth, as far as I'm concerned. Once the baby is out and calmed down, she hears the noise around her and responds to it. Mommy talks softly into her hair, Daddy coos into her ear, and all around her are proud voices. Simultaneously, mom initiates nursing, and language begins. It's more than words, it's touch, satiation, calming voices. She responds by suckling and falling asleep.
By the time she comes home from the hospital, she has control of her lungs, and crying ensues at whim. In no time, the parents can distinguish sounds from her, they know if that cry is from hunger, wet diapers, discomfort, or heaven forbid, boredom. And she has also learned that more parental attention ensues when her noises are loud. As her vision improves and she learns to focus on faces and external sounds, her crying becomes less frequent. She develops an interest in the world around her.
At some point, she realizes that she can make noises herself. Not screams of hunger, but coos, purrs, gurgles and burps. These all are first steps into language and most babies are heavily into it by 4 weeks. If the parents are attentive, she learns that calming noises are attention getters, and uses them to bring the smiley face of mommy into her orbit.
How many of us have held a baby and made popping noises against her tummy, cheeks or neck? All are steps into language acquisition and each noise is welcomed by the baby. When we see their first smiles, it's usually the result of some auditory/visual stimulus. By the time she has reached three months, she's a smiling, gurgling noise machine. She still cries, but more often than not, her noise of choice is that of play or joy.
As she plows her way into development, she develops the ability to steady her head, semi-sit, and in some cases roll over. This is almost always accomplished by chortles of glee that are returned by the parent or care giver. She becomes more responsive to her own activities by making noise, it just isn't for the benefit of attention.
By the time my two were each six months old, they were language pros, even though neither one could utter a word. They crowed with delight, cried in pain, demanded feedings and elicited love. Words weren't necessary, although they recognized their own names, their own special arrangement of sounds, and responded to it. Say a baby's name, and she will kick for joy, crow for attention, or root for the breast. This is the real language of babies.
There are theorists who think that newborns should listen to certain kinds of music, etc. But I think that the most important sounds a child hears, from the beginning, are the sounds of security, love, nurturance and domestic normalcy. The sound of a clothes dryer, the hum of a dishwasher, the lilt of conversation between other members of the family are all parts of language acquisition. Play a little Mozart (or Chopin) in the background if you wish, but keep up the steady stream of "baby-talk" as well.