Behind Enemy Lines Infinity Ring Series ; Book 6 by Jennifer A. Nielsen Copyright Year: 2013 ISBN-13: 9780545387019 Available format(s): VOICEText (H) Add to Bookshelf What's an Audio Format Audio format refers to the way an audiobook is recorded. Not all audiobooks have the same formats. Classic Audio: A human reading an audiobook without the text displayed. VOICEtext (H): Human narrator with text that you can follow along with as it reads. VOICEtext (S): Synthetic voice with text you can follow along with as it reads.