Do you play any other sounds around that same time? I'm pretty sure only 1 sound can be played at the same time when creating sounds with triggers. And after testing it, it seems like playing 2 or more sounds at around the same time causes ALL of the sounds to stop playing.
Anyway, I attached a map with a neat little setup you can use for playing sounds. It's probably not the best solution but if you're already going to use a dummy unit anyway then i'm sure it's fine.
I create a dummy unit and order it to cast either Locust Swarm or Windwalk. The Locust Swarm ability is for Looping Sounds and the Windwalk ability is for One Shot Sounds.
You can copy and paste these abilities and set your own Effect Sounds. Also change the duration of Locust Swarm to be however long you want the Looping Sound to play.
Note: Locust Swarm was the only instant-channeling ability that I could find that would play a Looping Sound. If you know of another one you can use that ability instead.