I haven't made a pseudoscience post in a while, so here goes. Even we normal folks can hear things in our sleep. Most people put this in practice daily by setting an alarm clock which normally wakes us up. Obviously, we don't process all sounds when we sleep. Ambient noise usually doesn't successfully wake us up, and it's not like if you fall asleep while watching TV do you usually remember hearing what was said while you were snoozing.

Anyway, some time ago, an undergrad at Johns Hopkins and her faculty professor gathered evidence that the areas of the brain that process our hearing while we sleep are the primary auditory cortex as well as the frontal lobe, an area of the brain associated with vigilance. They believe because of this triggering of vigilance, it's why people can sleep soundly (I always thought "soundly" was a strange adjective to describe uninterrupted deep sleep) when there's ambient noise, but certain sounds that you react to can wake you up. So here's the pseudoscience application: Superman is extremely vigilant toward the sounds of people in trouble. That's why he hears them when he sleeps instead of tuning it out.