mkv2mp4 is a commandline utility, written in Python, which allows conversion
of video files in the Matroska container (*.mkv file extension) containing
H.264 video to be converted into a format which the Xbox 360 can play.

It differs from other similar tools in that no video transcoding is
performed; the video is passed through untouched and just the audio is
transcoded if necessary.  This means that the conversion is much faster
(the whole process being quicker than realtime on reasonably modern
machines), and more importantly that there is no degradation in video quality
(which makes it very suitable for HD video).