Why do rice krispies make sound?

Answer 1

All the tiny air bubbles trapped inside the rice krispies begin bursting when you add the milk, and the small variation in air bubble size's burst with a slightly differant noise, making all the snap crackle n pop sounds you hear.

Em, nice as that sounds I actually think the reason is a little less friendly. I'm pretty sure it's due to the high salt content in Rice Krispies because you get the same reaction when you add cold water to salt

Answer 2

The rice is toasted which makes it puff up, when milk is introduced the puffiness collapses making the snap sound.

It's why when they come out of the box they are rounded, but after milk the are flat.