I'm surprised they're talking bout having the landing pad 10km off shore--it seems like that wouldn't be far enough for noise reduction, both outgoing (rocket engines) and incoming (sonic booms).
Doubling the distance only gives you a 6db drop. So, the best approach is probably to have structures in the way to redirect sounds. Which should make a dramatic difference.
This mostly deals with ignition and static firing where the rocket is not moving. Rockets have ridiculous acceleration, but create a lot of noise on the ground. I assume being kind of noisy for 5-10 seconds a few times a day is not that big a deal.
Assuming ~20 stories aka 200 feet is reasonable at 100 feet per second acceleration aka just over 3 g's in 2 seconds the base of your rocket is just over the 20 story building and your doing 130 MPH. Even still that building is still going to be blocking a lot of sound as the fire is below that level.
7 seconds after launch your (5 after clearing the building) your doing 477 mph and have traveled 1/2 a mile. At 12 seconds your doing 820 mph and have traveled 2 kilometers up.
PS: Falcon Heavy is apparently ~160db vs 120db for thunder, which you can barely hear at 10km. So, it's going to be noticeable at 10km, but not that noticeable.