Where does music downloaded from Google Play go on my phone?

New Nexus 6 with Lollipop. I downloaded some music from Google Play, but cannot for the life of me find where it is on the phone. I've searched through the entire thing using Astro, and can't find my music. (The Play app sees it, but I want to make one of my songs a ringtone, and apparently with Lollipop, you need to copy your file into a certain location for it to be available.)