$ sudo apt-get remove libavcodec56 

Uninstall libavcodec56 including dependent package

If you would like to remove libavcodec56 and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove libavcodec56 

Use Purging libavcodec56

If you use with purge options to libavcodec56 package all the configuration and dependent packages will be removed.

$ sudo apt-get purge libavcodec56 

If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.

$ sudo apt-get purge --auto-remove libavcodec56 

See Also,