Alex forgot to mention that while doing this you must stand on one leg, tip your head back and try to touch your nose, while saying the alphabet backwards.
If you actually want to do this, and I can just picture the scenario under which this might make sense, then you can take a track into Audacity (
www.audacity.sourceforge.net), select the track, and select "effects / reverse." If it's your intention to reverse your entire library, then this might take a while.
Reversing tracks can help you find the darnedest things. For example, the well-known fadeout on "Strawberry Fields Forever", where it appears that John utters ominously, "I buried Paul," thereby igniting a popular rumor that Paul dad died. With the advent of digital audio, you can now hear clearly that it is not John Lennon's voice, but Tiny Tim, gowling in a hushed voice, "I'm married, Paul."