You can't play them on the Sparkle app, but you can play them on your device with iTunes or music player!
Not currently. At present, the Sparkle app is a "player" for stories in the Sparkle subscription service. Only subscribers to the streaming site will be able to listen to stories on the iOS app and website player.
However, the Sparkle Store allows you to be able to purchase and download any story you’d like to play on your local player. You can play them directly on your computer or transfer them to any mobile device to be enjoyed on that device’s player. (Just like you do with songs purchased off of iTunes.)