If you're outside you can hear them. If you're inside your room they are muffled, but you can still hear them "pop." But they're not as loud as the Electric Water Pageant is while inside your room.
ETA: Thought I should explain that we always stay on the Bay Lake side of the Tower and not the MK side.
Thanks everyone! I was thinking about switching to a MK View room at the Contemporary. DS (5) is still scared of fireworks because of the noise, so I wasn't sure if they were really loud. Sounds like you can hear them, but they aren't super loud; he would be ok with a pop or a small boom, and he would probably even like it.