... when someone's voice is overlapping with a dozen other voices in the room: I can't understand them.

If this is your only or primary problem I would recommend having your hearing checked before you start on the neuro stuff.

The comments to this post and most of the other literature I've read assumes that the problem with poorly social people is that they're afraid, not sure how to carry out a conversation effectively, or make poor decisions when confronted with social dilemmas.

Anecdotally, my experience isn't like this at all. I'm pretty good, maybe even better than average, at talking to people in one-to-one conversations, at home, at cafes, on the bus, before class, and pretty much any time other than at deliberately social events. But at bars or parties, the constantly shifting conversations of dozens of people trying to all talk to each other at once at a mile a minute, about nothing in particular, in a loud and overstimulating environment completely discombobulates me, and I usually end up either ignored, unable to break into a conversation more than once every few minutes, or just plain bored with having nothing to say but the same small talk everyone else is making.

Maybe I'm atypical of non-social people, but I also give a bit of credence to the possibility that all this "not knowing how to give the right reply in a conversation" stuff is what neurotypical people imagine being bad at socializing must be like, the same way hicks sometimes deal with non-English speakers by speaking English words really loud and slowly because they can't imagine what it's like to genuinely not understand English. But I'd like to hear from other non-social people to confirm.

(I got a 23 on the test)