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