Yea, what's your audio setup. Sound through TV? 5.1? 2.0? There could be a myriad of answers depending upon your other equipment.
Assuming just TV sound, I'd suggest checking the settings of the bluray player and make sure it is sending a stereo downmix to the TV. It could just be getting front R/L and not the center channel, which is where the dialog is.
If it is a 5.1 and you have an AVR and it has audyssey, run audyssey. Then if dialog is still a problem use dynamic volume which will supress special affects frequencies and enhance dialog. (This will also help in any speaker set up running with an avr.)