Reality is that most attractions at WDW are loud and use the element of surprise with loud noises quite often. I find all the 3-d movies pretty loud with burst of louder moments. Many of the shows, though I don't remember Nemo using loud noise too much, many of the indoor ride through rides are loud as well. Things they can see ahead of time are good starters. Dumbo, Aladdin, Carousal etc. Maybe try to watch the utube yourself so you can warn your child when there may be a startle coming.