My DS#1 jumps alot at loud noises, fireworks, shouting, everything.
Some tips I have found that work really well for these kiddos while touring:
Take some Post-Its to use to cover the toilet sensors while your child is in there. The toilets are REALLY loud. Take it from someone who had to check them on occassion! If your child can hold their bladder for a bit, scout out where the Babycare centers are in each park, they have smaller toilets that aren't quite as loud, more like the ones in school!
For Wishes you can watch the Fireworks at several locations around the 7 Seas Lagoon (and I think have a better show sans crowds!) The dock at the Poly, Narcosse's at GF, The California Grill both of those locations will pipe the music in, and the dock at the Poly will have it on the speakers but faintly. There is also a balcony at the Contemporary that you can ask any CM about and they can point you to it for a wonderful view!
The 4-D shows can get a bit aggressive for my eldest as well, so we covered his ears or had him do this and keep on his glasses and the muffling was just enough to help him. He got scared a couple of times but I think it was more from the special effects than the noises in those cases.
You can take ear plugs for your child and they make some with a vinyl cord attached to both so he can hang them around his neck while just walking, plus it makes it easier to PULL out!
We did not find Turtle Talk to be Loud at all. In fact our sons were both yelling with the best of them. I think it is more the SPEAKER and BOOM sounds that really get to them. Childrens voices they are naturally used to especially if they are in school, day care, or preschool during the day.
Hope this helps and your family has a wonderful time!