Good afternoon from Spain,
I have checked Kaffeine, Noatun, Kaboodle, Codeine and KMplayer as video players for Matroska contained Theora videos. In Kaffeine and KMplayer I got the audio playing but in any of them the full video was supported.
I wanted to know if there was some support being worked in format support in any of the video players and which was the most promisings.
As a workaround, I changed the container of the videos to OGV using ffmpeg built-in "copy codec" which got the video working at least in Kaffeine (KMplayer has some issues changing the size of the window and only a part of the video is showed).