$ sudo apt-get update
$ sudo apt-get install cmus-plugin-ffmpeg 

cmus-plugin-ffmpeg is installed in your system.

Make ensure the cmus-plugin-ffmpeg package were installed using the commands given below,

$ sudo dpkg-query -l | grep cmus-plugin-ffmpeg *

You will get with cmus-plugin-ffmpeg package name, version, architecture and description in a table

cmus-plugin-ffmpeg package basic information: