I too am having an audio/video sync issue. I purchased this laptop this week, so it is a fresh install. In each browser I was having this issue. Netflix, Hulu, Amazon Video, Twitch, YouTube and Plex(local stream); all having audio about 1 second before the video.
I have disabled all browsers "Use hardware acceleration when available" options, and still no luck.
I also installed the latest driver and firmware updates from the links provided earlier. The computer is also up to date on windows updates.
After several tests, I think i found a temp solution(until they fix it).
This may be a solution: One thing I did change, was the hardware acceleration on the Audio driver itself, and this seemed to help. I have now watched around 1 hour of video without seeing the delay, but I wasn't paying as close of attention as when I first noticed the delay. To disable the acceleration, go to your sound options, and select your playback devices. double click your speakers/headset properties and navigate to the advanced tab on top of the window. Remove the Check box for Allow Hardware acceleration of audio with this device. Hope this helps.
I truly hope Microsoft can fix this issue as for something fresh out of the box, should not have this ridiculous of a problem. Considering this thread is 3 months old from when it was first brought to attention, doesn't seem like they're going to fix it any time soon.