In the Services add-on “Plex LinuxServer.io” I have set var/media/Toshiba Ext/Media for both the sources (movies and TV shows). Toshiba Ext is my storage device Media is a directory (which contains subdirectories called Movies, Music and TV shows).
I then went to the Plex webpage (192.168.1.40:32400/web in my case), added a library for music and linked it to the Music subdirectory when it asked for the source.
Hope you get therewith this.