Thanks to Bonnie King <[log in to unmask]>, I think I have solved this problem

Here's the sequence that appears to have worked for me:

yum remove libmatroska
yum remove libebml
yum --enablerepo=epel install libebml-1.2.1-1.el6.i686
yum --enablerepo=epel install libmatroska-1.2.0-1.el6.i686
yum --enablerepo=epel --disablerepo=atrpms --enablerepo=rpmfusion-free-updates install vlc

Now to test it out more fully....
Malcolm