I have the same problem. Despite turning AV sync on and adjusting it to -5, the audio is still not in sync with the video.
https://www.avforums.com/threads/lg-e6-youtube-audio-sync-issues.2081173/
http://www.avsforum.com/forum/40-oled-technology-flat-panels-general/2621057-youtube-lipsync-issues-e6.html
As you see, all E6 users have problem with YouTube WebOS 3.0 app. Please update this app LG. A TV this expensive shouldn't have this kind of issues.