My husband and I were just talking about this yesterday. We have twin infants so we opted to skip going to fireworks this year. However we can see them from our balcony. They are literally 4 miles away and they were still loud. Not painful loud, but 4 miles and we could still hear them clearly. Disney fireworks are never that loud. All we can assume is that Disney either buys or designed special ones that just aren't that loud. We were walking around the world showcase during illuminations last month and the babies weren't disturbed in the slightest.
That said, they made some change to the speaker system in the great movie ride when the upgraded it last month. We found it way too loud and covered the babies' ears with our hands. The narrator is a fine volume, but the CM is blasting. I assume they will get that worked out though.
It doesn't hurt to carry ear plugs if you are worried about it. If you are really super sensitive (I can't handle concerts or bars, but I am totally fine in disney) or have a sensory disorder, you will likely need them. Id say 99% of people don't need them though.