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GarageBand is Apple's music editing software, available as part of iLife or packaged on its own. It is made to be an easy-to-use music and podcast creation tool. It also ships with a fairly large number of loops and software synthesized instruments. Ufortunately, Garage Band lacks the ability to reverse audio clips, but with some freely available tools, we can take the music file out, reverse it, and put it back in Garage Band. And if you're really upset that you can't do it in GarageBand itself, you can complain to Apple.
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The Solution: Audacity
Because GarageBand can't reverse audio files, we'll use another program: Audacity. Audacity is a full-fledged, free, open source audio editing program that is fairly lightweight and happens to have the ability to reverse audio. If you want to edit mp3 files with Audacity, download the “LAME MP3 encoder" (on the Downloads page for Audacity). We don't need that for this tutorial, but it's useful.
It's also possible to use Audacity on MP3 files (if you just want to reverse a sound file) and for many other interesting effects - it comes with a rather large effects library, and there are many more plugins available online. Give it a try; it can do some things Garage Band can't.
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Get the Music from Audacity, Reverse it, and then back to Garage Band
The first thing to do is locate the music you want to reverse. If you don't want certain tracks to be in the reversed sound file, mute them in GarageBand. Then go to the Share menu and select “Export Song to Disk..." Uncheck the “Compress" check box; although the intermediate file will take up more space, it will come out full quality. Save the file somewhere you'll remember it.
Then open Audacity and go to the File menu, select “Open..." and open the .aif file you just saved. Select the whole file (or press Command-A) and then click on the Effect menu and select “Reverse." From the File menu, select “Export as WAV..." and save this file somewhere memorable. You can also trim the file to just the clip you want in Audacity before reversing it; you might only want a clip from it.
Find it in the Finder, drag it to the GarageBand window, and you're finished! Your sound file is now reversed, and you have it as a clip to play around with. Happy composing!