Do you want the sound to loop by itself, or do you want it to play only after a certain trigger was activated, or something else?
We need more info for that.
In the worst-case scenario, you can use a logic_timer, which will call the ambient_generic's PlaySound() function, every duration of the sound (e.g. every 5 seconds).
The ambient_generic's flags should be like this:
Play everywhere - up to you
Start Silent - check
Is not looped - check
Lastly, I'm not entirely sure about this one, but ambient_generic should only be able to playback mono sounds.