I noticed it when I was a teen. I assumed that I had hearing loss due to playing in rock bands and attending loud clubs. I started religiously wearing earplugs, hoping (as a musician) to guard what remained of my hearing. When I stopped studying music and went to graduate school, I had to sit at the very front of every class, or I couldn't "hear" what the professor was saying.
Concerned that I might need hearing aids, I had my hearing tested several times. Each time, my hearing was completely normal. I was concerned that I was just gaming the hearing test. When I was little I also had my hearing tested. I learned to anticipate "tones" by watching the face of the person giving the test -- looking for "tells," microexpressions or other evidence that I should be raising my hand. (Sociopaths must be difficult to diagnose for certain things because of this.) At my last hearing test, several years ago, I insisted that I face away from the examiner who was already in another, darkened room separated by glass. I passed with absolutely normal hearing. Still I doubted the results, wondering if my acute sense of timing was causing me to hear tones in what I knew would otherwise be an uncomfortably long silence.
The puzzle was that I did not have a hard time hearing in general. I took several acoustics and sound recording classes at university and had an exceptional "ear" across the sound spectrum. It was just speech that I had a hard time deciphering. Not language. My reading comprehension has always been off the charts. Verbal language.
My friend's niece learned to read when she was just one year old from (shockingly) those "your baby can read" DVDs. Someone opined that the niece might be hyperlexic, characterized by an extraordinary facility with written language, frequently paired with a difficulty in understanding verbal speech. Hyperlexia is associated with the autism spectrum (as with other language issues), with some experts believing that all hyperlexics are autistic. I don't think I'm hyperlexic. I show no real signs. I do think, however, that my inability to decode verbal speech has less to do with my ears and more to do with my brain. Brain wiring? Attentional problems? Whatever it is, it seems to not affect music cognition, but that's another thing shared with the autism spectrum.