DIAGNOSIS This is an audio sync or signal processing bug in webOS, often triggered by certain sound modes or ARC/eARC conflicts.
FIX Disable all audio processing. Go to Settings > Sound > Sound Mode > set to 'Standard'. Go to Sound > Additional Settings > turn off 'Auto Volume Leveler' and 'Sound Sync'. Go to Sound > Advanced Settings > Digital Sound Out > set to 'PCM'. Power cycle the TV.
IF THAT DIDN'T WORK Reset the audio driver. With the TV on, press and hold the MUTE button on the remote for 10 seconds. The TV will momentarily blank. This forces a re-initialization of the audio subsystem. Check for a software update immediately after.
HEADS UP If using an external soundbar via HDMI ARC, the cutting out can be an HDMI handshake issue. Try a different high-speed HDMI cable on the ARC port (HDMI 2). Ensure the soundbar's firmware is also updated.