This challenge has a stereo audio file with the left and right out of phase. Mixing them together produces the difference, which contains morse code.
$ sox ecstasy-atb-music.wav -c 1 out.wav
If you can decode morse by ear you go it! Otherwise use rscw or do it by hand:
sox out.wav -t ub - | rscw -f 720-750 -w 11
This will give you a string of numbers that represent ascii characters. Use a decimal to ascii converter