And Yes, I can distinctly hear them better when I do so.

(Oh, and Sam — If you read this, thanks for the excellent info — layman or not.)

You probably do have a slightly more sensitive ear than other people, but some of the situations you described aren’t so much cases of hearing things that are inaudible to others, but your brain “choosing” to hear what others don’t. Locations that people would call “quiet” are usually not quiet at all – there are all sorts of sounds all around you in your house or on a quiet street or even in the woods. It’s easy to appreciate the difference between the ambient noise levels at night and during the day, even though in both cases there aren’t actually any sounds you could pinpoint and name.

The hum of the computer monitor or the mosquito repellent could certainly be in a frequency range that your brother or your wife can’t hear due to physiological reasons, but any music played back in a store is in the audible-to-all spectrum by design – your brain is simply better at picking up vague patterns from background noises. This seems like a plausible explanation to why you have difficulty focusing on something like a person’s voice – there’s something behind the ambient noises that’s competing for attention.

Lots of people are incapable of having a telephone conversation if there’s a TV or a radio playing at moderate volume – I personally am not bothered by it at all. Given your ability to hear things others can’t, it’s quite possible that people often talk to you in an environment that has some sound equivalent to a TV or radio blaring in the background, and the auditory center of your brain zeroes in on that while you are consciously trying to focus on the person who’s talking to you, so you don’t actually consciously register the source of the distraction.

This would also explain why it comes and goes – when it’s gone, there’s no distracting sounds in the ambient noise, and vice versa. Also, there’s a variety of factors that could affect the acuteness of your hearing at any time – alertness, tiredness, stress levels or even something more arbitrary like the light levels in your surroundings or the time of day.

The “comes and goes” factor, on the other hand, seems to rule out any physiological defect in your ear, because something chronic and progressive wouldn’t go away, and acute temporary hearing loss due to excessively loud noises wouldn’t be likely to affect only a limited frequency range.

Then again I’m not a doctor so I might be completely wrong. Just my interested-in-neurology layman’s two cents.