I know I'm not the only one who hears this problem: Regardless of the TV (or stereo amp through which I route the audio on one of my devices), the sound quality is regularly pretty dismal with movies and some special-effects-heavy series. WAY too much in the way of background sounds and explosions and not enough of what should be the center channel dialogue. Yeah; I know there will be some who will recommend sound bars but my experience (and others if I go by what I see posted here) is that a soundbar just makes the loud sounds more evenly distributed left-to-right and the center channel voice volume is still too low.
Regardless of having a soundbar or not, I have to pump the volume so far up just to hear people speak that, when the background music (or special effects) come on, it wakes the neighbors.
The Roku software should have some sort of "center-channel boost" (and I mean a LOT of boost) so the "master" volume may be kept considerably lower.
Just as an example: When I transcode video containing AC3 5.1, I drop all the audio except for the center channel and the resulting show (or movie) may be played at a comfortable level (with all the dialogue clearly audible).