Well, Woody, I'm not sure that Superman would make a fireball in any case, when he is either falling or flying very fast through the atmosphere. Meteorites create fireballs when they fall through the atmosphere because they heat up so much that they partly or completely vaporize. That is not something that would happen to Superman, since he is invulnerable... on the other hand, his cape should burn up every time he flies really fast.
And Superman really should create sonic booms. There is no excuse for him not to. Anything sizable moving sufficiently fast through the atmosphere will create sonic booms, and to my knowledge there is no way you can avoid it.
But like Olympe pointed out, the laws of nature as we know them simply don't allow a living being with a human's weight, size and build to fly under its own power, much less fly incredibly fast. If we accept that Superman can fly at all, and fly unbelievably fast at that, then there is no reason why we shouldn't believe that he can fly without burning up his cape or creating sonic booms. It's called suspension of disbelief, and we need it in order to enjoy all fantastic stories about what is scientifically impossible.
Ann
