Go into the World Editor and click on the Sound Editor (looks like a speaker). From here you'll have a list of folders, find a sound and import it. Once the sound is imported, double-click on the added sound file in the list (it should be empty on a new map).
In the double-click dialog you'll see there are a few fields, one of which is Channel. Rather than having integer values, you'll notice there are easier to read string values. I think there is some type of priority given to different sound channels, or perhaps two sounds from the same channel cannot be playing simultaneously. Not sure.