The code can be found on the copyright page, and Mike pointed out that you don't actually have to buy the Kindle ebook to see the code. The copyright page is at the front of the ebook which means you can get the free sample and then see the code.
Mike thought that was a mistake (the code shoudl be at the back of the book), but I disagree. It doesn't matter where the code is - so long as you can return the ebook. If the code had been in the back of the ebook, someone could buy it, get the code, and then return the ebook. There's slightly more work involved, but they still get the free MP3s.
And besides, this is a simple promotion. There's no way to really make sure someone bought the book. This means that you have to accept a certain amount of cheating, and I'm sure Derek already figured that out.