I would say a house with an outer brick skin is thicker and denser then thinner lengths of timber. Although this is an open ended question. Because for example a log cabin would probably share similar noise reduction quality's
Bricks have millions of small openings in them. This is how sound is muffled. Mufflers, small music rooms, and silencers on guns all follow the same design. If a small hole exists in an area, then only a small amount of noise can easily pass through.
Do this millions of time and even a gun shot can sound like a purring kitten.
In Building Acoustics we have to analyze, among several parameters, the sound isolation to air conduction sounds, sound isolation to percussion sounds and the reverberation of the spaces. Given the question that is placed, I believe that the question refers to the sound isolation to air conduction sounds. In this case, we have to consider the Law of Mass Theory, which basically says that the heavier the element is the best sound isolation provides to air conduction sounds. So, under normal conditions, we have a better sound isolation in brick houses than that for houses of wood, because a brick house has more mass in the external envelope elements therefore better sound isolation.