You may be seeing this bug in 12.04 - https://bugs.launchpad.net/ubuntu/+source/gst-plugins-bad0.10/+bug/973014
To test you can move a specific plugin to a .bak, then try totem on the file. If it then plays you're affected. The moving of the plugin is NOT a solution though many have used it as a temp workaround.
To TEST on 64 bit install
sudo mv /usr/lib/x86_64-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so /usr/lib/x86_64-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so.bak
To revert the above move
sudo mv /usr/lib/x86_64-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so.bak /usr/lib/x86_64-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so
To TEST on 32 bit install
sudo mv /usr/lib/i386-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so /usr/lib/i386-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so.bak
To revert
sudo mv /usr/lib/i386-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so.bak /usr/lib/i386-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so
Source:Askubuntu
And if you're facing that problem , you'll have to use a different player. Like VLC.