Summary
You can slow down an imported audio track in GarageBand iOS by tapping on the track, going to settings, and adjusting the speed.
1
Regular audio files do not support this function, but you can use the Sampler to import the sound and then change the pitch to change the speed, although this will also change the pitch.
2
To slow down a song, open the Tempo Tracks from the Track menu or use the key combination ⇧⌘H and drag the control points in the Tempo track up and down to change the tempo for selected measures.
3
You can also click underneath the tempo button and adjust song speed as needed, with increasing tempo speeding up a song and decreasing tempo slowing down a song.
4
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