If the magecraft behind the Castle of Separation was based on sound, then it might be better to characterize its howl as a wave suffused with magical energy.
A wave of sound inaudible to humans. Even if it was filled with magical energy, it was still a completely unknowable existence to an ordinary human. By changing the magical energy and wavelength of the sound, it could be changed from a wave that disabled Magic Crests to a blast of darkness that destroyed the magus’ opponent.
If that was the case,
"…Add!”
“Gotcha!”
How did it look to the beast, I wondered, when my blade sliced its attack right out of the air?
While Add had already begun his transformation, now the cage in which he was held opened even further. With a hazy glow like a Will’O’Wisp, their shape changed dramatically.
A form that anyone would recognize, of a tool used to harvest.
A shape to collect souls.
A Grim Reaper’s scythe.
“Ahahahahahaha! What a great feeling! What a great night! It’s like an all you can eat buffet!”
Slicing apart the wave of magical energy, the mouth engraved on the scythe laughed wildly.
In the night sky above, the moon was just short of being full. Below, the crescent moon of Add’s blade was beautiful, the image of it slicing apart the waves of illusion burning itself into my heart.
Case Files I, Chapter 5, Part 3