From the article: "Amazon, which offers a free streaming music service with a limited catalog to subscribers of its Prime shipping and video service, did not respond to a request for comment about the new, full-fledged music plan."
They... already have one? What is this headline about?
I guess they're just going to spin-it-off so you can access it without being a Prime member.
My guess is that it will be unlimited listening for a flat monthly fee, with a larger selection than is available for free listening with Amazon Prime.