I think there is already a sound for the sword. Something like sword.ogg (or maybe with a different extension). You can try to replace that file with your sound, it could work. But that only works for the sword that is implemented in the engine. You can always fully script your own sword.
Edit: in your case, if you don't hear a sound, maybe the problem is that the event item:on_using() is not being called for some reason (maybe something is missing in your code).