In the "old age", meaning music was available on records or cassettes, you could play a music backwards. It's not an easy thing to do, but doable. You play the media, physically, backwards.
There are stories that you can hear certain kind of messages, weird ones, if you play certain song backwards. I don't know whether it's true or not, but certainly it is interesting.
Now everything is available in digital format. Songs are (mostly) in MP3 format. Now, the question is ... is it possible to play a digital song backwards? It should be possible, right? All you have to do is write a simple program to do it? :)
[the answer: use audacity. now, let's find some interesting songs ... :) any recommended ones?]