the sound of a crash
訇 represents a loud, crashing sound, like a sudden impact or explosive noise.
訇 combines 言 (words/sound) inside 勹 (wrap/enclosure), suggesting a sound that bursts out or is contained. The exact historical development is uncertain, but it likely represents a loud noise.
Words (言) wrapped inside a bursting enclosure (勹) create a sudden crash of sound.
For コウ, imagine a loud crash that sounds like a gong: the 'ko' of gong cues コウ, and the noise echoes inside the 勹 enclosure.