If you're wondering what the differences are between MP3, CAF, and other file and data formats for audio, check out http://www.raywenderlich.com/69365/audio-tutorial-ios-file-data-formats-2014-edition, a detailed explanation on all the different types of audio. It's not necessary for this book, but if you're trying to save space or wondering if you can use certain audio files, this link will be helpful.

We're going to be using MP3 in this chapter (and subsequently in the rest of the book's content) as it's a very common format, as well as a format that's supported by OALSimpleAudio and the iPhone natively.