Basic audio editing follows the familiar text editing method. First select then apply some action (delete, cut, copy, move, or paste). GarageBand also has some new very userful methods for modifying audio segments.
GarageBand uses non-destructive editing. This means that after you save an audio recording (File>Save) you can always get back to the original saved audio file.
The basic methods for navigating within a track: click anywhere on the ruler to place the playhead at a specific moment. You can also drag the playhead to any point on the ruler. To jump to the beginning of the project, press the Return, Enter or z key any time. Press the Spacebar to start or stop the playhead.
Selecting a segment
If you click once on an audio segment in the Timeline, you will highlight it (it will be a darker color when you click it, or purple in this case below).
If you click and hold, you can drag the segment left and right along the timeline. You can also drag the selected segment up or down to other tracks.
Split or join
Move the playhead to a point where you want to split an existing single segment into two segments. Then select the segment. The segment will turn blue as in the image below.
From the GarageBand menu select Edit>Split or use the keyboard shortcut Command-T. This splits the segment into two pieces, and renames the segments in their upper left corners, in this case, "test track" becomes "test track.1" and "test track.2".
Click off of the segments to deselect both, and then click to highlight one segment.That section can be moved or deleted independent of the other section.
Segments can also be joined together. Highlight two or more consecutive segments by holding Shift while clicking each consecutive segment. Then choose Edit>Join from the menu or use the keyboard shortcut Command-J to combine the selected segments.
Undo an edit
To undo this, or any other edit, simply press Command-Z or choose Edit>Undo in the menu (Command-Z is usually much faster, so remember this shortcut). Again, remember to save your project periodically (Command-s).
Shorten or Lengthen an Audio Segment
You can shorten or lengthen a segment by hovering the cursor over the bottom left or right corners of the segment until a cursor symbol with triangles on either side pops up. When that cursor appears, click and hold the mouse button and drag the segment boundary to lengthen or shorten the segment.
Using the Editor Window
If you prefer, you can also modify an audio segment by double clicking it to open the audio waveform in the Editor at the bottom of the screen, or pressing the button with the scissors icon.
In the Editor view, you can click and drag to highlight specific sections of audio, which you can then cut, copy, and paste (Command-X, Command-C, and Command-V, respectively) or delete (simply press Delete).
In the editor the waveform of the audio is visibly larger, allowing for more fine tuning.
Note that in the bottom left corners of both the Editor pane and the GarageBand Window, there is a white slider. This is the Zoom Slider, and it allows you to zoom in and out on the soundwaves for even finer editing.