Close-on-Hide External Plugin
Well, I took up the challenge and wrote an external plugin that does just this - clicking the close button (window "x" button) really does mean quit.
In the default way of working, Ubuntu has a specific patch to ensure that when music is playing, clicking the close button minimizes the application.
The reason for this is that some people like music to be playing, but they don't actually want to see the application visibly on the desktop. Control of rhythmbox then passes to the Sound Indicator.
However, for those that do not like this behaviour, you cannot by default quit the application via the close button when something is playing.
how to install for RB3.0 and later
sudo apt-get install git
git clone https://github.com/fossfreedom/close-on-hide
Start rhythmbox and enable the "close-on-hide" plugin in the plugins menu (Tools - Plugins menu)
If you are using RB2.99 and earlier - edit the .plugin file in the close-on-hide folder and change the line
Loader=python3 to read