I just imported an mp3, but how do I get a .wav or .aiff in? The wav and aiff I tried to add just becomes horrible when I drag them into the window.
From the GarageBand Help:
Adding an audio file from the Finder
You can add a loop or other audio file (in AIFF, WAV, or MP3 format) to a song by dragging it directly from the Finder to the timeline. You can drag the audio file either to a Real Instrument track or to an empty area below the existing tracks.
When you add an MP3 file by dragging it to the timeline, the file is converted to an AIFF file which is stored as part of the song.