Some audio content vendors like Tumblebooks require that the book be read online on an Internet connection - there is no download capability for offline listening.
Other vendors allow you to “stream” the content (i.e., listen while you are online) or to download it and listen “offline” on your computer or handheld player.
Some vendors offer users the ability to listen to streaming snippets of audio books. That way, your users can decide if they like the writing style, topic, and narrator’s voice before making a checkout/download decision.