For increased clarity/intelligibility, I would move the three front speakers forward several inches, so that they are forward of the TV screen, not behind it as they appear to be now. In illustration, if you cup your hands on both sides of your mouth and sing, your voice is muffled. When you remove your hands from around your mouth, the intelligibility of your voice improves.
If you still have trouble hearing speech after you have moved the center speaker forward, by all means raise the volume of the center to where you can hear comfortably.