The Booktrack Studio is now open to all readers and authors to play around with. The Beta trial is over. They just launched yesterday.
Booktrack uses a simple interface for adding three types of sound from a large library of clips (music, ambience, effects). They give you access to an extensive library of high quality recordings (including natural settings, actual city streets, etc.) and they can be layered multiple times to create more complex sounds. As a musician, I'm finding it lots of fun.
The way it works is that you just highlight passages and link them to sounds (and the link is color coded green, pink or purple to indicate the type of sound file linked. There are considerable controls over fade-ins, fade-outs and overall volume. Check it out!
http://www.booktrack.com
To listen any browser or phone works with the web-site or app. To edit your own files, though, Google Chrome is necessary.