When they showed babies pictures of fish with either an adult saying "Look at the toma," a made-up word for fish, or a series of beeps matched for tone and length with the words, the babies who heard the words formed the category of fish, but the ones who heard the beeps did not, as measured by then showing them pictures of a fish and a dinosaur side by side and measuring how long they looked at each.
There's something special about the human voice. I heard something on NPR yesterday about exposing babies to different sounds, including words in a language the baby has never heard before, they early on develop a preference for the human voice.
In a more practical sense, there's something special about talking to babies. One reason richer kids start off preschool ahead of poorer kids is that they have heard more words, more total words and more different words. This study shows it also helps kids cognitively.