CashMusic.org is an open source system that does many of the things that I’ve been trying to do. I wanted to create a generic system for musicians to sell their music on a simple website, but there is no need for me to get back to that project.
I wrote a plugin which will appear, I hope, on the WordPress.org plugin directory any minute now. I will set up the documentation here as soon as that is done.
The Plugin allows anyone with a WordPress installation to also install Cash Music and then include the elements in a WordPress page or post via a shortcode.