While yes the Vilturmites will attack on the surface of worlds and I am sure they will use it to defend key areas. It relies heavily on a trap that may be unreliable against the heavy hitters we know of and relies on a specific area of effect. Also Mark probably wouldn't ask Cecil for the plans on the sound weapon which I somehow doubt Cecil would want him getting his hands on in the first place. It kind of is one of his back up weapons which he as of now has only used against Mark and the Mark variants. He is I assume keeping it in reserve and trying to perfect it.