You are asking this question because you cant hear what in the world people are saying to each other during a movie or show right? let’s see if my solutions work for you.
Is your center channel speaker located on your TV stand? If yes move it towards the leading edge closer to you. If it’s too far back the dialogue coming from the center speaker will reflect off the table and mess up the dialogue.
Is your center speaker at ear level? make sure you mount your center channel speaker at ear level. If its too low or too high and you do not want to remount it just angle the speaker towards the proper ear level so it projects the dialogue directly to your ear holes.
The content you are watching may be mixed poorly. So make sure you try several different movies only after you have the center channel positioned correctly. If you have a friend or a showroom that will allow you to test a couple movies on their setup you can play it there. Find out which movies you know for sure have good dialogue clarity then test it on your setup.
You may want to turn up the volume 2 or 3 dB higher just to see if it fixes the low dialogue problem. This may or may not fix your problem. If it does not then there may be some issues with your speaker function or speaker quality. You may have issues with your room setup too. You may have too many reflective surfaces.
Keep your center channel speaker away from hard surfaces like tables, glass, mirrors, tile and stone. These surfaces will disrupt the flow of dialogue coming from your center speaker. They reflect sound just like light will reflect off of them.
What size drivers does your center speaker have? 4 inch drivers seem to be an optimal size but before you waste your money buying a new speaker only to have the same issue make sure you do everything possible to fix this low dialogue issue first.
I read somewhere that center speakers should not be set below 120Hz but that may be for certain speakers. If yours is set below 120 Hz make sure you set it higher. Make sure it is not firing any bass either. Turn the bass response all the way down. Maybe you need to adjust the Hz lower than 120. Experiment.
Have you turned on the Dynamic EQ? This will automatically turn up surround speakers which in turn can drown out your center. You will have to decide how much of an offset you need between surrounds and center speaker.
The most expensive problem that you may have is poor acoustics. Some rooms are not designed well for home theater sound. You may need to add some sound absorbing acoustic panels. You can make them yourself or buy some but the big question is? If you spend your resources will they work?
Maybe you can experiment with panels and panel placements with a couple and if you notice improvement you can get more. Here are some articles regarding acoustics. Home Theater Acoustic Treatment Layout and How To Make A Sound Proof Room For A Home Theater
We hope this helped. Good Day!