Party Wall
Interior/party walls are very often not insulated and offer little in the way of blocking sound transmission. Dense insulation provides significant mass and effectively attenuates airborne noise. Adding an additional layers of drywall/QuietRock w/ Green Glue will provide further attenuation.

Floated Ceiling/Floor
Airborne noise such as conversation, television & light music can be effectively attenuated by insulating empty joist cavities between floors. Impact/vibrational noise such as footfall & bass heavy music is most effectively treated from the floor above with an underlayment affixed to the subfloor or from the ceiling below with isolation clips/hangers.

Doors
Typical hollow-core doors have an STC rating of 15 and solid-core doors about 27. Replacing/retrofitting standard doors w/ sound deadening materials in addition to air sealing frames/thresholds will significantly improve this common weak link.

Windows
For single pane windows, the STC Rating is most likely between 26 and 28. The difference is the glass thickness and how air-tight the window is. Louvered windows can be less than STC rating 18 in many cases. Building custom removable plugs is one way to deal with exterior noise.

Flanking Noise
What is flanking noise? Flanking noise is noise reaching a room by an indirect path. For example noise reaching the room above your home theater from some route other than through your ceiling, noise reaching the next room by a route other than through the wall, and so on.