Experimental is the key word. As of today there is no defined standard for OPUS in Matroska. I believe that Opus support is stable now, perhaps I'm wrong. From: http://lists.matroska.org/pipermail/matroska-users/2013-September/006883.html 2013-07-05 Moritz Bunkus <moritz at bunkus.org> * mkvmerge: ...