I tried 3 Mi Boxes and all had issues streaming videos via Google Cast. 4K, 1080p, 720p videos all had audio and video sync issues. 4K and 1080p were so bad the video would play in slow motion. I returned it, and tried two other boxes, all had the same issue.
I have a Vizio 55" 4K TV. It's so disappointing, the Mi Box worked well otherwise. I installed the firmware update on each also, but did not fix the issue. Hopefully it gets fixed soon.
Some background info. I've tried casting from a Nexus 6, 6P and Moto X, same issue. My internet speeds range in the 300-350mbps download, 5-10upload. This was around the first week of October, don't know if Google Cast has had an update since then.
I've reported this issue and support stated it should be resolved in an upcoming update.