Books have no audio
Not all languages have audiobooks, and not all books are audiobooks.
Audiobooks are available in 31 language courses and only 100+ books have audio in each language. If you see the audio icon on top left corner, it means that book has an audio, otherwise it's just a regular book.
Next to the audio button on top left corner, there is also a "Translate to English" button, and that particular book would have the audio in English.