…fwhooff…

It's the noise you always hear in the movies when an atom bomb hits, that dull air-rushed thud that's even more baffling because the word "bomb" is attached to it. It's almost as if the very fact that it's the most deadly thing you can imagine makes it unnecessary to have all the noise you normally associate with other deadly stuff like dynamite, shotgun blasts, and cannon fire. After all, you're LOOKING at a mushroom cloud by that point, which is the visual symbol of total annihilation, so the "death wrapped in a Kleenex" kind of noise is meant to underscore that, I suppose. The bigger the cloud of death, the more likely you are to get that softer, quieter sound.