Oxford University Press offers on their website. While you won't get all 80 units, you can download 2–3 complete units (covering topics like "Numbers and time" and "The world around us") for free, no code required. Search for "Oxford Word Skills Basic sample PDF and audio."
Sites like Mediafire, 4shared, or random blogs offering "Oxford Word Skills Basic audio rar/zip" are often: Oxford Word Skills Basic Audio Free Download
I’ve seen a few people asking about a of the audio files for Oxford Word Skills Basic (the red book). Oxford University Press offers on their website