Q: How can I hear different parts in a piece at the same time I am recording another part in Garageband?

Monitoring

I hope I have understood your question (??) And, I think I would be right in saying that in GarageBand, you can lay down multiple voice tracks. By adding a new track for each ‘harmony’ voice, you could perfect each track to your heart’s content.

Monitoring is another word for ‘playback’ and you could listen to each track individually, or at the same time through, either your earphones, or auxiliary (monitoring) speakers. Do this by “muting” the tracks you don’t want to hear.

I have been using the Focusrite interface: iTrack Solo unit, that has a built-in ‘monitoring’ component. Check out this link:

http://us.focusrite.com/answerbase/using-the-itrack-solo-with-garageband

But, you may not have the iTrack Solo unit. If this is the case, you would need the GarageBand Gurus to come forward with suggestions.

Have fun

To be able to monitor a track make the Input monitoring button visible in the track header.  Go to the "track" menu, then select "Track Header", then "Show Input Monitotoring".  Enable the button. 

By default, inut monitoring will be disabled. If you use this option, you will have to wear headphones to avoid feedback.