Amazon and Audiobooks.com sell digital audio books online, while volunteer organizations such as Project Gutenberg and LibriVox offer free audio books in public domain on their websites. Google and Apple dedicate a section to both free and paid audio books in their mobile marketplaces as of 2015.
Most of these websites provide online streams or downloads for their audio book selections. The majority of the audio book files are in MP3 and AAC format, which makes them compatible with many media players and mobile devices. Audible and Audiobooks.com advertise their own mobile applications, available on Google Play and the iOS App Store, for convenient listening.