As you have correctly mentioned, the term Nacelle generally refers to a number of sections around an aircraft engine (In the context of turbofan aircraft engines). This generally the structures formed by the Inlet, Fan Cowl, Thrust Reverser and the Exhaust.
The nacelle does perform the basic function of a housing for all these subassemblies which means it contributes to protection of the engine from the elements and the environment from the engine (Say a blade failure). But its major function is to provide an aerodynamic body which can house the mechanicals of the engine, the pneumatic ducts, fuel lines and electrical wiring.
The nacelle can therefore provide noise reduction by reducing the drag-noise which would be otherwise generated. This is through the aerodynamic design and the various features built into the subassemblies such as Acoustic Liners used extensively througout the nacelle which convert acoustic energy into small amounts of heat. This is a major source of noise-reduction in modern aircraft engines.
Exhausts are the biggest culprits when it comes to aircraft engine noise and as such, the exhausts are designed to create a 'mixing' area where hot air from the exhaust is mixed with ambient air to reduce the exhaust noise.
I guess I'm just trying to say that the nacelle is formed of these various subassemblies and therefore their individual contribution towards noise reduction is very important. As far as a quantitative study on the reduction of noise, I could not locate any as therefore can only manage to answer the first part of your question.
Sidenote: Modern Inlets also use Laminar-flow design (777X for example) which contribute to immense fuel savings and very minor drag-noise reduction (Probably negligible).