Hi,
I wanted to listen my Plex music on Linux and didn't find any way to use multimedia key.
So I created a music player with full desktop integration, using nuvolaplayer.
I tested it under Ubuntu 15.04 with standard Plex music. I don't have a premium account so it will probably does not work with premium Plex music library, sorry.
To install the application simply follow nuvolaplayer beta build installation instructions.
I hope you will enjoy this player as much as I enjoy it every day. Don't hesitate to comment for any help/question.
Code source of the integration script : https://github.com/tiliado/nuvola-app-plex