Your reader is for Ebooks, i.e. books in formats suitable for READING only and that is all you can do with the 100 Ebooks the reader came with. What you are thinking of are AUDIO books. Ebooks = you can only read them (visually), cannot listen to them. Audio books = you can only listen to them, cannot read them.
However, since your reader is also an MP3 player and the most popular format for audio books is MP3, you can download audio books to your PC and then transfer them to your reader to take them with you and listen to them wherever you are. From your PC you can also copy the audio books to an SD memory card and then put that card on your reader. Here are 3 ways to get audio books:
1. from your public library (if you live in USA)
2. see the two links I have attached to my answer.ww
LibriVox free public domain audiobooks Audible Try it free Audiobooks on your phone tablet or computer Audible com