12pm Midnight is closing time for all bars, 7 days a week. Only Oceanside and I&I have any activity after 10pm. They are both pretty rocking on the weekends, and pretty quiet during the week. The Sunset View (aka "Disco" or "the Club") is open from until 4am ONLY on Fridays and Saturdays. It is soundproofed, although I would recommend not staying directly across from it.
I would say 95% of the hotels/houses on the island will be as quiet as you want. It should not be a huge concern.