Anne Fernald and her students have spoken several times at the Hearing Seminar about infant's ability to understand prosody.
Infants understand the prosody of speech, long before they understand the words.
Interestingly, our machines understand the words, and haven't a clue about the prosodic message. Gerald McRoberts and I did a study last summer to see if we could teach machines how to understand basic prosodic messages. We collected more than 500 utterances from adults talking to their infants. I'll try to bring a demo. We used several pitch and formant measures, and a multidimensional Gaussian mixture-model discriminator to perform this task.