Usually when playing a video the audio starts fine, no matter where I start from in the file. After about 4 or 5 Minutes I can see lips not matching. Depending on the length of the video I can sometimes see the video and audio are seconds apart.
I'm using windows 10 64-bit, and have restarted my chromecast regularly. Youtube casting works. Tab casting works. All my video files play fine locally but the problem ones are all avi and mkv files, many of which seemed to work fine in the past. The problem seemed to start for all videos I play (I have very few in mp4 format) about 2 months ago. I haven't had any hardware changes in that time period.