DIAGNOSIS: That's not a defect, it's a setting. You've got Night Sound or Speech Enhancement cranked way up in the Sonos app, which boosts dialogue and crushes everything else.
FIX: Open the Sonos app, go to the Settings tab for your Arc. Under 'Sound', tap 'Speech Enhancement' and 'Night Sound' to make sure both are toggled OFF. While you're there, run Trueplay tuning again if you can (you need an iOS device for this). That'll rebalance all the speakers for your room.
IF THAT DIDN'T WORK: Check your TV's audio output settings. If it's set to 'PCM' or 'Stereo', the Arc isn't getting the full Dolby Digital or Atmos signal it needs for the immersive effects. Switch it to 'Auto', 'Passthrough', or 'Bitstream'.
HEADS UP: Some streaming apps, like Netflix, have their own in-app audio settings. Make sure it's not set to 'Basic' or 'Stereo' there either.