None but a mute roo is a godsend surely? If he goes totally mute I'd breed from him with the objective of trying to get his mute genes into his offspringGreat pic btw.
The no crow collars work by putting pressure on air sacks on the side of their necks. Because they can't fill them properly they can't crow as loud. I would feel the sides of his neck to check for any sort of lump that could be growing and preventing the filling of those air sacks.