I was just on a 2 day Inaugural cruise on the Symphony. Though I had a balcony room on that cruise, I was on the Harmony last October and had a Central Park View. The Harmony and the Symphony are essentially the same ship, tho the Symphony is a bit larger. On the Harmony I was on the 9th deck right above one of the restaurants. It was nice and pretty quiet, but the awning of the restaurant obstructed out view below a bit. Plus you can see into the cabins across from you and need to pull the curtain when changing, etc.
Although I didn't have a boardwalk balcony on the Symphony I can vouch for the fact that it is pretty loud pretty late into the evening. The latest aqua show starts at about 10:30, and there is a sports bar right next to the aqua theater and an arcade right next to that. Also, last year on the Harmony I was on my company's National Conference with about 900 colleagues. We had our Halloween party on the boardwalk deck and I know that was loud. Again, I didn't stay in one of those rooms.
Good luck and have a great cruise!