In principle, the dialog should be primarily on the center channel track; a good home theatre decoder and separate speakers will generally let dialog be heard properly. Sometimes in the name of "art" they have background music/sound effect cranked up too loud - and this swamps the center track even if you have a good surround system. (This is well known, and at least some DVD player manufacturers have a mode that automatically increases center channel volume by 3 dB - so it's common enough that manufacturers are including compensations in their hardware!)
When they mix the standard audio track, they should make proper allowance for viewers without a home theatre setup, but I've noticed that many times, they don't . . . they've gotten lazy or careless in manufacturing the DVD.
This also applies to TV shows ("NCIS" is an example) where, if you're watching it on a standard or 2-channel stereo TV, the dialog is sometimes swamped by "background" music or effects.
About a month or so back, the TV critic in the local rag mentioned this was one of his pet peeves, and in a follow-up a few weeks later, said that judging by his mail, it was a peeve of many who'd read his column, too.
So you're not alone in noticing this.
