My wife has complained of this same problem when we watch not only DVDs but VHS tapes. Some movies come across fine with the right mix of voice (coming from the center speaker) and action sounds (coming from the surround speakers). Others you have a hard time hearing the dialogue out of the center speaker because of louder music, etc out of the the surround speakers.

To be honest, I enjoy the movies as they are originally cut. But to keep peace I use the stereo speakers in the TV to "augment" the center speaker by turning them up just enough to boost the center sound without drowning out the surround sound. I find this to be easier than constantly adjusting the surround speakers through the receiver.