$ sudo apt-get update
$ sudo apt-get install libavcodec-extra57 

libavcodec-extra57 is installed in your system.

Make ensure the libavcodec-extra57 package were installed using the commands given below,

$ sudo dpkg-query -l | grep libavcodec-extra57 *

You will get with libavcodec-extra57 package name, version, architecture and description in a table

libavcodec-extra57 package basic information: