rumble, explosion, blast
軣 centers on a deep, rolling sound: a rumble, an explosion, or a blast. The meanings all share the idea of a loud, resonant noise.
軣 is a variant or simplified form related to 轟, which uses three 車 (carts) to represent the rumbling of many wheels. The exact historical development of 軣 is uncertain, but it likely shares the same cart-rumbling origin.
A single 車 (cart) on top rumbles loudly, and the 冫 below looks like the ground shaking from the blast.
For コウ, imagine a cart rumbling so loudly it sounds like a deep 'koh' echoing: koh -> コウ, the sound of the explosion.