Hi, there have no any 3-rd party package.
Even paid package.
I have already spend a month to survey this topic.
You have several way to implement.
1.Using Custom Media Foundation to build custom decoder, but really complex(You may need write some native C++.)
2.Using App contract to call other support FLV/MKV(Like "PressPlay Video"、"K-Lite for Windows"、"VLC media Player"
http://msdn.microsoft.com/en-us/library/windows/apps/xaml/hh779672.aspx
3.Call external WebPage to play, you may need write a website can receive flv source, finally I using this method to solve this problem, not perfect but can use.
Try look this http://www.ntex.tw/flv.html?www.ntex.tw/MSIFLV.flv
