Really interesting stuff! This sort of correlates with my special interests as well (music technology, sound design etc.) So a really long post from me too.
I always heard things as a kid that nobody else seemed to pick up. As a blind person, people always used to brag about how good my hearing was. Later in life they would correct people and say "he doesn't hear better, he's just more attentive to the sounds he hears because he can't see.."
Here's the kicker though. I am actually not more attentive. And on any given day my hearing, or my attention to the sounds I hear, is probably average, if not below average. Only in specific situations do I really notice that I'm picking up things that really aren't expected.
I tried the party test, and I could not hear the man claiming he was a guerilla at all. I heard something being repeated, but by the time I heard that, I was already filtering, and vaguely trying to focus on the women because that's what I was directed to do. Of course when the man was mentioned explicitly, I think I almost died laughing when I heard the man calmly and nonchalantly declaring he was a guerilla 14 times (I counted twice to be sure). And I still find it amusing. I'll never unhear it now.
If I am an aspie, and I really don't know if I am or not, but if I am one, I don't have a major issue with overloading by sound. Not often anyway. When people are shouting or screaming it does get to me. But most normal sounds I can filter. When I was a child though it was a bigger problem. Unexpected sounds would frighten me. Anything that was loud, or rumbly or whooshy that could move was an instant threat and I would run screaming into my room and hide. Certain intonation patterns in people's voices would scare me because I associated them with being in trouble for things I didn't understand. And I also had more trouble adjusting to loud environments. But all of that has gotten a lot better. I've gotten really good at selectively hearing things. If it isn't catching my immediate focus, I can very easily ignore it. Which unfortunately became a problem in class when my teachers became background noise that my brain could easily just filter.
I do have audio processing issues with music though. If I'm transcribing a song, or trying to compose, I need absolute musical silence. A fan in the background is fine. Having a conversation with certain people who understand I'm concentrating is fine. People talking quietly in the background is fine when I have headphones on. A few beeps from an elevator or a cell phone or a computer is fine once I'm so used to those sounds that I can filter. But where it isn't fine is when I hear very musical sounds that aren't related to what I'm working on. Like if someone starts singing, I either have to stop or move to somewhere quiet.
The worst is when I'm composing something in my head, and right at the height of it, when I'm getting all excited, realizing this probably won't really come to anything because I'll forget all of this and won't be able to get it down, but still I want to bring my internal composition to a close... someone turns on the radio or TV and it's playing some tune. As soon as that happens, it's like pulling the plug on your computer without saving your work. Everything I had been thinking is gone in an instant without a trace, along with the desire to try again. I don't even have to be concentrating all that hard for my focus to get blown.
My piano teacher used to ask me how I compose things if I don't always have a piano on hand. I had to tell her that sometimes I rather wouldn't. At some stages during the composition process, especially if I have an idea and just need it to really spin around and let it be loose, a keyboard is useless to me. I can't reproduce things on it like I hear them in my head. The keyboard is useful for externalizing, solidifying, improvising detail, working out complicated twists and turns which I only hear outlines of in my mind. But I don't need to be at one to conceive my ideas, and it is often when I am away from one that I can start writing something that ends up truly making an impact on me when it's externalized. Maybe this isn't so unusual, but I've always found it fascinating.
Where it becomes a real problem is when other musicians are around. I was once working with a friend of mine who wanted the two of us to collaborate on an album, but we work in different ways. He doesn't have perfect pitch like me, and while he's a great musician, he can't hear things and know what they are without sort of picking them out first. So he asks me if I have ideas for a melody to go with chords, and as I'm trying to stimulate my musical mind, he starts clanking on his guitar, saying "oh that's cool," and a few seconds later starts describing to me what notes he's doing, or asking me to remind him of that sick chord I had come up with a minute ago. and I wanted to cover my ears because firstly I know what you're playing you don't have to tell me, secondly when you ask me to think, I need musical silence, so this collab session might not work so well.
I'm really not sure what to make of this though. Is it a normal musician thing or does it indicate some heightened sensitivity even in that context? Could it be some sort of APD? Hmm...