What is audio steganography?
Audio steganography is about hiding the secret message into the audio. It is a technique uses to secure the transmission of secret information or hide their existence. It also may provide confidentiality to secret message if the message is encrypted.
What is Elliot's way of hiding his secret data ctf?
The answer is that Eliot is hiding the data on those CDs. He is actually copying music to those CDs, then embedding encrypted information on them that only he can recover. So anyone who finds them will see and hear only audio and will be unable to find or retrieve the hidden information.