$ sudo apt-get update
$ sudo apt-get install liquidsoap-plugin-alsa 

liquidsoap-plugin-alsa is installed in your system.

Make ensure the liquidsoap-plugin-alsa package were installed using the commands given below,

$ sudo dpkg-query -l | grep liquidsoap-plugin-alsa *

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

liquidsoap-plugin-alsa package basic information: