Rice Krispies are made of crisped rice (rice and a sugar paste that is formed into rice shapes or "berries", cooked, dried and toasted), which expand forming very thin and hollowed out walls that are crunchy and crisp. All the tiny air bubbles trapped inside the Rice Krispies begin bursting when one adds the milk, and the small variations in air bubble sizes causes slightly different noises, making all the snap crackle n pop sounds one hears.
Nice as that sounds, (apart from the excess sugar) the reason for the noises could be is a little less friendly. It could be due to the high salt content in Rice Krispies because one gets the same reaction when one adds cold water to salt.