If you are a Mac user rather than a PC user, you must have Adobe Reader installed in order to hear the embedded audio files in these eBooks and PDF documents.
Mac computers open PDF files in an application called Preview by default, not in Adobe Reader.
Preview does not play embedded audio or video files in PDF documents.
Adobe Reader for Mac is available for free from this link.
Here are some instructions about how to set Adobe Reader as the default PDF reader on your Mac.
Once you have downloaded the file, save it to your hard drive and open it with Adobe Reader for Macintosh.
Some files also need the latest version of the Flash Player. Install this too if you are having problems.