Welcome to cruise-chat.
I've stayed in a Bridge Deck (deck 10) cabin on the Majesty. Don't worry about noise. It won't be a problem. Even on the balcony, I couldn't hear anything other than the ocean. Maybe an occasional sound of the poolside band playing in the afternoon, but that was it.
The Windjammer buffet restaurant is totally enclosed and noise from it is not a problem.
Seas can be smooth or rough any time of year. There is nothing about December that changes that fact. It is totally dependent on weather conditions and that is not predictable. Most likely things will be fine. That is the normal case.
If seas are pleasant then all cabins have a nice ride with little or no perceived motion. If seas are choppy you might feel it more if in a high deck cabin in the forward part of the ship, but to be honest the ride isn't much different anywhere else although cabins on the lowest deck in the middle of the ship actually have the most consistent and smooth ride to them. Thus the irony that the cheapest cabins can be the best ones in a storm if you are susceptible to motion sickness, and the most expensive cabins can be the worst for you if ill. However, in most cases you won't notice any motion no matter where your cabin is located. Ships all have stabilizers (think of the fins on a humpback whale) which are used to negate any pitching.