For challenge level 'C', the audio file should play the same forwards as it does backwards, as if it was being held up against a mirror.

For A and B, it is a lot more lenient, only using palindromic elements instead of the entire song being a palindrome. For example, maybe you have a melody going A, C, D. To make it a palindrome, you'd have it go A, C, D, C, A or A, C, D, D, C, A. That way, they are the same forwards and backwards.