The backports repository was enabled by default in the M12 beta, eemaestro did not check first to see if it was enabled in Synaptic and if the 1.0 gstreamer packages were available (they were), so now there's a duplicate entry in the sources. Firefox would also need the gstreamer 1.0 package for playback.
Since wheezy-backports is the only source of the 1.0 gstreamer packages, we actually don't have to specify where they come from with the "-t", either.
M12 did not ship with gstreamer playback set up correctly for all users, for that the "gnome-media" package needs to be installed, then run in a terminal
to test and configure gstreamer. But pulseaudio 5 from the M12 test CR might just get everything working automagically, too. It does on my laptop.