Diagnosis
This is a common surround sound mixing issue, and your soundbar's 'Night Mode' or 'Dialogue Enhancement' features are probably off.
How to Fix It
Use your remote. First, press the 'Night Mode' button. The front display will show 'NIGHT' for a few seconds. This compresses the dynamic range, making loud effects quieter and quiet dialogue louder. If that's not enough, press the 'Dolby' button to cycle through sound modes. Stop on 'MOVIE' mode, which is specifically tuned to enhance center-channel dialogue.
If That Didn't Work
The problem might be your TV's output settings. Go into your TV's audio menu and make sure any "Surround Sound" or "Dolby Digital" output is turned OFF. Force the TV to output stereo (PCM). The soundbar's internal processor will do a better job upmixing stereo to clear dialogue than your TV will with a garbled surround signal.
Heads Up
If you're using HDMI-ARC, some TVs have a separate "Digital Audio Out" volume or level setting. Make sure it's set to maximum or 'Variable' so the soundbar is getting a full-strength signal to work with.