If you want to import an .mp3 audio file to Garage Band and edit it, read the following.
If you want to import an .mp3 file into GarageBand, you need to convert the file to a different format.
Use iTunes to convert the file to an AIFF before dragging it into GarageBand.
To do this, go to iTunes.
Click on the iTunes tab on the menu bar at the top.
Click on Advanced.
Change the setting on “Import Using” to “AIFF Encoder.”
Drag your .mp3 file into iTunes.
Once the preference is set, right-click (or option-click) the .mp3 file then select “Create AIFF version.”
Drag the .aiff file to your desktop.
Open Garage Band. Create new podcast.
Drag the .aiff file into Garage Band into empty gray space below the jingle track.
It will import the file and you can edit it.