In response to a question on the FAWM forum here’s how to pitch shift audio files in Garageband
First select the track you want to work on
Click on cmd E to open the editor window at the bottom of the screen
Then click on the actual piece of audio you want to transpose (this will turn the audio from light purple to dark purple and the pitch slider from faint grey to bold)
Now move the slider by the amount of SEMITONES you want to transpose by (eg up a 5th would be +7, down an octave is -12)
The process will take a few seconds to take effect.
Some cautions. This doesn’t always work. I can’t figure out why.
You can (I think) select multiple files on a single track and transpose them all at once. BUT if you have comped them together as a single audio file (turning the file from purple to orange) it won’t work.
Hope that’s helpful.