This
tutorial will show you how to place sounds into
your level.
Presuming
you have your level/room created right click
somewhere over one of the grid windows and scroll
down and select target_speaker
from the target list.
Select
this entity by pressing shift and left clicking it
and then press N to bring up the entity box

What
you need to do now is select the sound you wish to
have playing from the sounds
description list.
In the
key box type the word noise and in the value type
in the path to the sound as above in the window,
to have the sound play constantly put a check next
to looped-on.
Check
out your level and you should have the selected
sound playing over and over.
I'll
update this page soon detailing the rest of the
options