Since updating to Windows 8.1 last week (perhaps this is a coincidence, although I doubt it), the XBox Video app no longer plays most content correctly. With downloaded or streamed videos (e.g. The Hobbit), the video lags the audio by up to a few seconds.
Other videos, not obtained through the store, play fine.
The videos played perfectly before the update to Windows 8.1. I am using the latest app version as of the date of this post. It was updated on Feb 14th; perhaps the problem came with the latest update. One weakness with an app approach is that there is no way to revert.
The most relevant specs are:
Asus M3N-HD HDMI mainboard with Athlon X2 5000 dual core chip
4 GB DDR2-800 RAM.
nVidia GeForce 9500 GT
Creative sound card/integrated HD audio both have the issue.
I changed video cards (From a 9600 GT) and sound cards, and it did not help. nVidia driver update did not help. No updates available in Windows update. "App repair" did not work.
Netflix HD content plays perfectly. It seems to be only the XBox Video app.