I firmly believe that Usopp went at least partially deaf during his time on the Boin Archipelago. The entire island would close like a flower daily, uprooting large trees and sending animals flying and screeching along with an earthquake-like sound. That sort of thing over the course of 2 years can really mess up someone’s hearing. This is why he’s been wearing those headphones since he hit the new world.
Usopp has always been handy with dials, so I believe he made some modifications on a pair of tone dials so they would record sound around him and play it back instantaneously, only amplified as a sort of stylish hearing aid. When he doesn’t have the headphones, he’s really good at reading a person’s body language (something he figured out after living with Heracles, who always keeps his face covered) and sign language. He’s also a fantastic lip reader.