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In Superman Returns movie, Superman claims that he can hear sounds from most (if not all) of Metropolis. Later he is shown to be able to hear and react to sounds such as alarms, car crashes, etc., while hovering well above the clouds.
How does he detect such far away sounds?
To answer your question, since it's now open, a normal human can hear Silbo (a whistled language) from up to 5 miles away. Sound really can travel that far.
However, lower frequency sounds take less energy to travel longer distances: an alligator mating call, for example, can be heard for miles -- and that's at frequencies humans can hear.
If Superman's ears are super sensitive, they are also likely able to hear frequencies that human can't -- and thus he would recognize these sounds by their longest-traveled frequencies that he could hear; this is consistent with the behavior of animals with greater-than-human hearing, like dogs responding to a dog whistle from large distances.
Superman has the ability to to expand his hearing, giving him the ability to hear things really far away as if they were right next to him. Anything that falls within his hearing range is instantly teleported to his ears. If this wasn't the case then if he were to overhear a conversation in China all the way from the UK then he would be hearing it 9 hours late, and not to mention that all the sound will have dissipated anyway, which is why humans can't hear sounds from long distances. He doesn't have to wait for the sound to travel to him, he can go to it. This is also how he can hear in space, all he has to do is expand his hearing all the way back to Earth and he will be able to hear things there.
