Diagnosis
This is almost always a software or audio processing glitch, not faulty hardware.
How to Fix It
Perform a full audio reset. Go to All Settings > Sound > Advanced Settings > Reset Sound. Confirm. Then, immediately disable any post-processing: set 'Sound Mode' to 'Standard' and turn OFF 'Auto Volume', 'Surround Sound', and 'Clear Voice Pro'.
If That Didn't Work
Check for a firmware update. Go to All Settings > General > System > Software Update > Check Update Now. Install any available update, as LG frequently patches audio bugs. After updating, power cycle the TV (unplug for 1 minute).
Heads Up
If you're passing audio to a sound system via HDMI ARC, the problem might be there. Switch the TV sound output temporarily to 'TV Speaker' to isolate the issue. If the cutouts stop, the problem is your ARC connection or soundbar.