I also think xine as a "multimedia" back end would be good.
Mplayer is very complete, too. It nearly plays all kind of videos without problems.
Mplayer has the advantage to be available on windows, too. As far as I know xine is unix, only. Or am I'm wrong?
But for both of them is the license problem: GPL...
Other - more low level libraries like ffmpeg have a less restrictive license like: LGPL. But if these kind of libraries would be used much more work would be necessary to really support most commonly used codecs and containers. Basically it would mean building something like xine with a more commercial friendly license. => Probably out of the focus of U++ and simply too much work.