Sounds like your system is not properly set up for home theater sound. What that means is that each speaker needs to be set up for size, distance and volume level.
Other things to check is whether or not your system is in a proper sound mode while watching a dvd or any kind of video source. Sony is known for using sound modes that say one thing but do not address proper listening mode. You should have your system set up for dolby digital or dts while watching dvds and not other types of stereo field effects.
If you cannot get your system to play at least dolby digital, it sounds like you used standard analog rca cables for your audio feed. You are going to need either optical or coaxial cables instead to pass a proper digital audio signal from your dvd player to your receiver.
I too known a Sony receiver but it is a ES model which is their very best.
I hope this answered some questions as to why dialog is so hard to hear.

There could be several other factors that are more in regards to equipment, wires and speakers, but at least try these few items first.

Good luck.