Kids are known to mature at different levels in their youths, although it's possible that the newborns were singing in their own minds or the babies imagined them singing. Dil could possibly talk later on down the road, but for now, thinks and responds almost like an ordinary baby. Another explanation can be that while the babies can communicate with Angelica and Susie (who are not much older), they cannot communicate with the adults. Therefore, newborns can possibly communicate with each other like the babies communicate with each other, but not with the babies since they are on a different level of maturity.