I just had a hearing test, and asked the examiner this very question. Was told that while there are occasional people with super-hearing, they are very rare and that condition is an anomaly unconnected with anything else. What people with autism are experiening is selective hearing, so its a matter of material coming through unfiltered. Relative loudness/quietness is a psychological effect.
In my own experience, i've had super-hearing (
) for most of my life. I can understand conversations heard several rooms away, I can tell almost precisely where noisy things are, locating their positions without seeing where they are first. Especially useful for birdwatching.
One thing that sends me into conniptions is multiple sound sources. If someone has two radios playing at the same time, my brain just falls over trying to make sense of both of them at once.
Sometimes I boggle at what a noisy place the world is. Most of the time, I figure its my problem, and I find one of my many sets of earplugs 