This is the latest version of the Matroska (http://www.matroska.org/) tools. Website is there: http://www.bunkus.org/videotools/mkvtoolnix/
Created attachment 17215 [details] /usr/portage/media-video/mkvtoolnix/mkvtoolnix-0.6.5.ebuild
mkvtoolnix-0.7.1 is currently in portage.