Given the Kindle's limitations in terms of formatting, and the variety of readers out there, this seems like the best strategy to me. It also means you can update the site whenever you want without having to worry about republishing the book.
Thanks. Exactly, and when I publish other titles I can ad them to the site in the same way a printed or eBook would have "other stuff available" at the back. I use the site as the source for the audio of 2 books at the moment and can ad more when I write more.
The only "downside" for me at least, is the tracks had to be all free, the copyright public domain with no commercial use (without having some kind of unique security code for each customer, not worth the hassle for me). No problem really seeing as the tracks are fairly pointless without the book, but it can also act as promoting it where people can listen before they buy no matter where (providing I linked the domain in the synopsis), as opposed to only on wholesalers that have that facility such as Amazon or Musicroom dot com.
Here's an example of an Amazon only kindle seller who has done this...
http://www.amazon.co.uk/dp/B004RIQAK0/?tag=absowrit-21
Providing the audio is integral to the whole product and fairly pointless on it's own without the eBook / book I'd agree that this might be the best option for popmuze with the benifit of no limits for the eBook format.
On the thought of passwords you could include a universal password in all the books that allows access to the portion of the site that allows the user to download tracks, while every other part of the site is promotional and informative available to anyone.