Audio is out of synch

When playing from my iMac (Plex Server) to my Smart LG TV some of the movies and TV shows I am playing the audio track is a second or so out of synchronization with the video. I know the movie audio is fine as when I play it via Apple TV and ITunes everything works fine. This has happened on several movies, e.g Everest was the delay between audio and video was a 3 or 4 seconds out and on Phoenix Nights its out by just under a second or so, either way, not good.

Any ideas pleaser. Tried searching the knowledge base for Audio Synch (etc) but nothing coming up.

Cheers

David