Looping Sounds
You can loop any sound by dragging.  Move the mouse to the left or right edge until the cursor turns into a left right resize cursor .

Simply click on the left or right edge of the sound, hold the mouse down and drag left or right.  In the example below, the user drags it to the right creating two and a half loops.

Each loop is represented by a triangle on the top and bottom of the sound.  

The sound will loop based on the snap settings. (If you cannot change a sound to a specific length it is probably because snap is on and you need to turn it off.)

Creating A New Loop
You can re-loop a sound or make a loop out of any section of any sound.  Click on the Sound Details Tab and use the Loop Editor to adjust the start and end loop points.

Alternatively, you can trim the sound and then click Make Into Loop from the Sound menu.  This sets the loop points to the start and end of the trimmed sound.  The Loop Editor will show more details.

Setting Exact Loop Length
You can set a sound's exact length by clicking on the Sound Detail Tab and editing the Length's minutes, seconds and milliseconds edit boxes directly.  This will allow you to easily change the number of loops in a sound.