That’s probably down to how the sound was mixed for the movie, and should be taken it up with the movie producer. 

What exactly are you watching? From what device and app? What audio format is displayed on the Now Playing screen in the Sonos app? What model TV do you have?

We have that problem with our Arc too, seems to be worse than with our Beams.

Sonos doesn’t offer a volume control for the center channel but you can use the controller to set “speech enhancement” that does help. Setting “night mode” also helps with some sources.

The producer / sound mixer is primarily at fault but Sonos could really help us out here with a center channel volume control, similar to what the Surrounds offer.

So I have solved this issue for the most part. First when running trueplay stay closer to your seating position and stay away from the soundbar. This will trick the system into thinking that it needs to boost the center frequency for a larger room.

I still have certain movies that are hard to hear dialog, because it's poorly mixed. For these instances I turn on night mode and dialog enhancement. Then turn the sub volume up. For my room size setting sub volume to plus 5 works nice. Then just turn the main volume up louder than you normally would if night mode was turned off. This make the overall sound better and with sub up it makes the bass better too. This also will help the center be more prominent for dialog and less of a difference in overall volume between quiet and loud scenes in a movie. 

Hope this helps. Please let me know if this has made and improvement for you.