AAC: Advanced Audio Coding. The file format used by iPods, Nokia and Android-based phones, and the latest generation of Sony Walkmans.
MP3: MPEG audio layer 3. A compressed audio format.
OGG Vorbis. "A completely open, patent-free, professional audio encoding and streaming technology with all the benefits of Open Source. - Vorbis site.
WMA. Windows Media Audio. A WMA audio ebook title, either DRM-protected or not digitally-protected, that has been optimized for download. WMA audio ebok titles are built using Microsoft Windows Media Audio format which enhances the sound quality for desktop listening , playing of CD copies (where permitted by the publisher), and portable device use.