I have this same issue and its been pretty frustrating.
LG 4kUHD 65"
Newer X1v4 box
Vizio 5.1 sound bar
Was originally hooked up with optical cable going from TV to Soundbar, essentially all HDMI sound signals going into the TV routed out to sound bar via Optical. That is until I set up HDMI ARC. After setting up ARC it seemed to be 'ok' and just like another poster; just wait about a few hours and the audio is so badly out of sync. I ended up having to restart the box, which is just stupid. Its about a 5 minute restart.
All other inputs, apps on tv, work satisfactory.
Netflix(on TV app) - no issues
Hulu(TV app) - no issues
Xbox One S syncs audio great on input 3
My Nintendo Switch, PS4 all sync good on input 2(Designated ARC channel, also has a HDMI in from 1080p receiver to Soundbar) all great.
All of these inputs route to the sound bar via HDMI ARC.
Any fix other than changing the input or restarting the box to fix this issue?? Nothing like a permanent band-aid.
** side note ** The audio sync on Comcast X1 v.4 box has NEVER lined up very well. Only tolerable at this point, it has always been slightly off by 8-10 milliseconds. I've tried LG TV A/V sync option along with the Vizio sound bar A/V sync with no success.
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