Yes, you can! To listen offline, you'll need to do the following:
1. Add a title to your library.
2. Then, in the library view, tap on the icon with the headphones & arrow next to the book you want to download.
Please note: audio titles are only available in the apps for mobile (smartphones and tablets) and not on the website. You can tell if an audio version is available by the little headphone icon on the book image. Once it's successfully downloaded, the button icon changes colour.