I have a Yamaha 5740 6.1 channel receiver. I had my home theater set up in the walls for speakers in each corner and a center channel over the TV. I also purchased a sub-woofer. I thought that that would account for the 6 channels (i.e., 4 corners, center channel and sub-woofer). But the dialogue in movies isn't crisp, particularly if it's whispered or if it's spoken when there's other loud background sounds. Am I missing a speaker? Does the sub-woofer NOT count as a channel? Am I supposed to have a center speaker in the back of the room that supports the center channel signal in front? Am I a complete moron? Answers to any of these would be a big help. Treat the last question as a bonus.
Go into setup on your receiver and TURN UP the center channel. For that matter, it sounds like you need to balance the sound on all the channels. Check your manual