The “Libro del Alumno” (Student Book) often ships with a CD stuck inside the back cover.
If you lost your original access code or bought a second-hand book without a code:
Why: Official sources provide correct, high-quality audio and respect copyright.
A: Difusión sells the "Libro del Alumno + MP3 descargable" pack. You cannot buy the MP3s alone for less than $10 because the audio is considered a premium part of the course.