GarageBand is powerful audio recording and editing software that comes as a part of Mac OS X's iLife software suite. With a few simple steps, you can slow down music in this program. It is possible to change both the tempo of a song you are creating using the software's many virtual instruments, or slow down the playback speed of a music file.
Open the GarageBand project file or MP3 you want to slow down.
Select "Project" from the list of options in the blue box at the bottom of the screen.
Click the number under "Tempo" and scroll to your desired tempo.
Play back the song to hear the effects of a tempo change.