DIAGNOSIS This is a common dynamic range issue. The soundbar is receiving a high-quality, uncompressed audio signal (like Dolby Digital) with wide volume differences, and its default mode isn't compressing it enough for your room.
FIX Enable Night Mode. Press the gear (Settings) button on the remote, navigate to "Sound Mode," and turn Night Mode ON. This compresses the dynamic range, making quiet dialogue louder and loud effects quieter.
IF THAT DIDN'T WORK Manually adjust the center channel level. With the remote, press the gear button, go to "Speaker Settings," then "AV Sync & Levels," and increase the Center Level to +3 or +4.
HEADS UP Ensure your TV is set to output audio in PCM or Bitstream (Dolby Digital) format, not DTS. The S800B does not support DTS codecs, which can cause odd mixing. Check your TV's external speaker audio format settings.