if you are trying to have it be a totally new sound to WC3 you will need to import the sound with the import/export manager.
i myself dont trust the looping effect sound... if i want a looping sound I'll either loop it with triggers and keep those blank, or I'll skip having it loop.
remember that if you have a trigger make a sound and the ability make a sound both sounds will go off at the same time... a generally unwanted effect.