Libavformat is supplied by libffmpeg0 on my system. If it is a prerequisite, it must be an indirect one unless your version of blender is different. I looked at the contents of my blender package and the "ldd" listing of both blender and blenderplayer.
I also looked at what requires ffmpeg0. I couldn't find how it was a dependency on my version of blender, Blender 2.42

As I said, from Blender's official site. It's only one Google search away :rolleyes: :

http://www.blender.org/cms/Blender.31.0.html