MythTV includes a built-in media player to play back your recordings, videos and DVDs. It supports an expanding range of video and audio codecs. It is created by pulling FFmpeg source, plus several modifications.
The Internal media player supports DVD menus, subtitles and multiple audio tracks. It supports playback from ISOs as well as normal DVDs. It is now the recommended player for DVDs and other file types over external players, such as Xine or Mplayer.
Just set the player command in MythDVD and MythVideo to "Internal" (capital I).