Audible is working on a brand new and much improved version of their audio book app for Windows Phone, and are in the process of rapidly releasing updates to the software.
Their latest update brings some significant new improvements, including the much desired variable playback speed.
New in version 188.8.131.52 is:
- Variable narration speed
- New and improved player interface
- Transparent tile option
- Performance enhancements
- Bug fixes
If you can tolerate bugs, you can download the current, fully working beta from the Windows Phone Store here.