There's a few different reasons why the sound appears loud at home:
-the audio transfer to DVD was done poorly
-the transfer was done with an emphasis on multichannel audio (5.1, etc.)
-you're watching a film with the audio set to a multi-channel audio when it's hooked up to a monoaural system/device
-the settings on your stereo/tv/whatever need to be tuned to maximum dvd audio playback
Sound design is an art unto itself really and there's lots of lit out there on the subject as it applies to home theater technology (if you're interested). Now that I've proved what a geek I am, back to the gym!