What are the differences of the Packages: libavcodec-extra-52, libavformat-extra-52, libavdevice-extra-52, libswscale-extra-0 in the Medibuntu and Multiverse repository?
Most of the packages in Medibuntu are now obsolete, but the main difference between it and Multiverse is that Medibuntu contains packages that are legally tricky or not child friendly (for example there is a system monitor application that claims to show a person undressing - I've never used it so I have no clue).
It was once the case that you needed Medibuntu for adding support for many proprietary formats, and for playing some DVDs etc, and even for Adobe Acrobat Reader. However, most of these have been moved into either the Partner repository or the Multiverse.
As for the FFMPEG libraries, they are essentially the same libraries with different compilation options for legal reasons. Medibuntu's version should not be necessary any longer.