primarily sell movies and music, video and audio utilities are an integral part of their marketing strategy. They understand that most people want to see and/or hear what they are buying. They know that using video and audio to show their merchandise to customers wins half the battle for them to get the customers to buy.
For websites that are focused on selling published materials like books and audio books, a few narrated passages or several sample chapters of the material encoded in audio files is a good way to encourage a customer to buy their product.
Furthermore, testimonials of previous customers have a better chance of being noticed than those that are just encoded in text. Most people usually go to a website for a specific reason. For example, one person wants to buy an independent video but he/she does not know which independent video vendor