flv is not a supported out of the box by the Windows Media Foundation (what the player framework uses internally). But you can extend support for other formats such as flv by building a
custom media foundation extension
(a very advanced task that requires C++ skills). Additionally, Windows 8.1 introduced a new MediaSource object where you could do the same work in managed code (easier IMO but still very advanced).
Or,... you might be able to find a 3rd party that has already done this work and could license it for use in your app.
fyi: Here's a
list of supported video formats in Win8
. (Look for the section: Video and audio support for Metro style apps.)
Sorry, I wish I could say it was supported natively by Windows (and therefore the player framework).