How do I import files into GarageBand on iPod touch?
To import audio files into Garageband for iOS, connect your iOS device to your computer. Then do the following:

On your computer:


Open iTunes
Click your device in the left-hand column
Click "Apps" in the bar at the top of the right-hand pane
Under "File Sharing" select "Garageband" in the "Apps" column
Click "Add..." in the bottom-right of the "GarageBand Documents" area
In the dialog that opens up, select the file(s) you want to transfer over
Once you've selected the file(s), it transfers over to your iOS device.




On your iOS device:


Open GarageBand
If you're not already, switch to the multi-track view by tapping the  button
In the upper right-hand area, you should see a loop icon with the number of newly transferred files on top of it, like this:


Tap this icon. It brings up a screen like this:


Just like the screen says, you can tap and hold and drag the desired audio files into your track listings. They each appear as Audio Recorder tracks:




That's it!

There is one caveat: GarageBand for iOS is limited to a maximum of 320 "bars" for the length of a project, and it does not appear that it's possible to bypass this. If you need to edit audio that's longer than that, you will need to use another app.