Sound Lock sits in your system tray and lets you set the maximum volume for your entire computer, so no matter how loud things may get, you won't wake up your neighbors. Just install it, click on the system tray icon, and move the slider down until you reach your desired level. When you watch your movie, the really loud parts should be lowered to the same level as the quiet parts.
The catch? It's only compatible for watching content on a Windows computer or a home theater PC, so Mac users will have to fire up Bootcamp or Parallels/VMware Fusion, while viewers watching via cable, satellite, DVD/Blu-ray, or streaming media boxtops are best served by their HDTV's built-in "night time mode" for Dynamic Range Control (the Playstation 3 has the option to average out the decibels under the Video Settings).