It depends....
What kind of asset is it? (You can tell by looking at its type in Content Manager.
If it's a track object, like a signal or switch stand, you place it on the track in various place. When the train drives over it, the radio chatter is played. If it's a scenery object, you place it as you would a tree, house, or some other similar object. The radio traffic is played randomly without any kind of trigger to it to cause the sound to be played.