DIAGNOSIS This is usually caused by the TV's Audio Dolby Digital Plus output setting conflicting with internal processing, or an issue with the eARC/ARC function even when not using a soundbar.
FIX Change the digital audio output format. Go to Settings > Display & Sound > Audio output. Set Digital audio out to PCM. Set eARC mode to Off. Set Pass through mode to Off. Next, disable sound-enhancing features. Go to Settings > Display & Sound > Audio output > Advanced settings. Set Sound mode Sync to Off. Set A/V sync to Off. Restart the TV.
IF THAT DIDN'T WORK A system software bug can cause this. Boot the TV into Safe Mode to test. Turn the TV off. Unplug it. Press and hold the physical Power and Volume Down buttons on the TV simultaneously. Plug the power cord back in while holding the buttons. Release when you see the Safe Mode logo. If the audio is stable in Safe Mode, a third-party app is likely causing interference. Uninstall recently added apps.
HEADS UP If you later connect a soundbar via the eARC port (HDMI 3), you will need to revisit these settings. Set eARC mode to Auto and Digital audio out to Auto 1 for proper surround sound pass-through.