When 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 (doing a lap or two of the entire city flying above the sea 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.