When I say "missing dialogue," I don't mean to imply that no sound is coming through. More than likely, all the sound is coming through each speaker equally in a 4-channel stereo. The whole point behind surround sound is that it is meant to create a sense of spatiality between yourself and the screen. For instance, the center channel is supposed to create the impression that the dialogue is jumping right of the screen (where people are talking).
Right now, although you have speakers surrounding the listener, you do not truly have surround sound, as each speaker is supposed to carry a distinct signal that is different than the other speaker. This is what you're missing, the individually encoded sound bits meant to be carried uniquely by each individual speaker. If you don't believe me, borrow (or purchase and return) a decent center channel speaker and subwoofer, get your receiver to play in Dolby Digital mode, and sit back and watch a movie. You'll notice an appreciable difference between before/after. Yet, to each his own... If you are satisfied with what you have now, why bother...