Despite most of what I've read on the internet, it is possible to get the Xbox video app on Windows 8 to open MKV files by following the steps below
First we need to add the video app to the 'open with' list for .mkv files follow the original guide here
or just enter the following in a admin command prompt
reg add HKCR\.mkv\OpenWithProgids /v "AppXhjhjmgrfm2d7rd026az898dy2p1pcsyt" /t REG_BINARY /f
There is now an option for the video app in the open with menu, however the app will only open not attempt to play the video. Contrary to popular belief however it is possible to get the app to recognise other formats
Download and install the Shark007 Windows 8 Codecs here
. After the install you will be displayed with the settings menu for the codec pack, select the MKV tab and uncheck the option 'Disable the DivX Media Foundation splitter'.
The video app will now recognise mkv files, however (this is where the "sort of" comes in) most files just throw a format not supported error. if you switch to the now playing screen you will see it list the name of the file, and the length if you try to play the file.
The problem lies in the DivX MKV Media Foundation splitter, it will only play certain profiles and codecs rather than connecting with the entire media foundation library. I have gotten files with mp3 audio to play with no video as per the attached screenshot.
So it is possible to expand the supported formats of the video app, the pressure is now on DivX and the community to make more Media Foundation splitters and codecs available
As a final note, don't expect subtitle support for MKVs anytime soon, the reason it was never supported is because the media foundation video renderer [EVR or Enhanced Video Renderer] doesn't allow for external modifications to the video output (e.g. subtitle overlay)