The difference between Music and Sound folders aren't the looping or not looping once started, but preload or not.
All files inside Music will be avaialable within stream (so it won't delay game start), while all files inside Sound will be available right when game starts, because it's downloaded with contents. (at least for me works like this)
So, to loop, once you Start/Play sound, just choose Looping -> looping on the popup, or, make a condition to set the tag sound to loop.