in the editor.Prerequisites:
The Basics
Overview:
Adding ambient sounds (background sounds that loop) is quite easy and is a must
for that finishing touch.
Expand Actor -> Keypoint, select AmbientSound, and add one to your map where
you want the source of the sound to come from. It will look like
this: in the editor.
Chang the Browse menu to SoundFX and press the Load button. Load up the Ambient.uax sound effects package (even if you already have a few Ambient sound effects listed). Choose an ambient sound effect (you can listen to it by hitting the Play button) and select it. Pull up the properties of your AmbientSound item in your map and expand the Sound category. Click in the AmbientSound value and hit the Use button. That's the most important setting, since, well, that's the sound it will make.
The SoundRadius is another important value. It determines the
range of the sound. It's not in Unreal units though, so be sure to
use the Radii View from the Actors menu (in the overhead 2D window, pull down
the editing window menu bar by hitting the 
button if necessary). The SoundVolume value should be clear
enough. :)
NOTE: Every "Actor" has a set of Sound properties, so if you have something in your map that you want to act as the source of the sound, you can just set up its ambient sound properties. You don't necessarily need an AmbientSound item.