While flying as Superman at hyper speed, he goes through stages where after an 
amount of time of continuous flying he gains a burst in speed. He accelerates 
and becomes faster and faster. If you fly for long enough (flying a lap or two 
around the entire city above the sea is recommended), he eventually breaks the 
sound barrier. It is about the fourth jump in speed. You will know this has 
happened when the sound of him ripping through the air is suddenly silent. This 
is because Superman is traveling faster than his own sound.