I've seen numerous people wearing small swimmer's ear plugs. I don't know how much they help,but they must offer some protection and they are very compact to carry all day. One of the loudest places to be during Epcot's fireworks is in the Boardwalk area - as the noise bounces off the Swan/Dolphin. For a slight improvement at Epcot, you can stand on the far side of the pathway and sit on the curb. You can actually see pretty well. A small thing that might help slightly (desensitizing) is to go on the Mexico ride right before Illuminations starts as there are light fireworks (almost no sound) in the ride. One we came out of Mexico and the fireworks were just starting and that seemed to help in our family - as the ones in the ride were a way to see 'fireworks' that weren't scary.
Magic Kingdom's fireworks can be heard all over the park.
You can see the Epcot fireworks from the Marriott World Center. This may not work for you, but if you have a room on the correct side, you can see the fireworks farther up is a better view) - with minimal sound.