This is an example of such a specification:
<stream basetime="npt:0"> <import contenttype="video/x-theora" src="fish.ogg" start="npt:0"/> <import contenttype="audio/x-vorbis" src="fishsound.ogg" start="npt:0"/> <import contenttype="audio/x-speex" src="comments.spx" start="npt:7"/> </stream>