sound of a large crowd of people
哬 represents the sound of a large crowd of people, like a noisy gathering or uproar.
哬 combines 口 (mouth) with 何, which likely contributes a phonetic element. The exact historical development is uncertain, but the structure suggests a character related to vocal sound.
The mouth radical 口 beside 何 suggests many mouths asking 'what?' (何) all at once, creating the noisy sound of a large crowd.
For カ, imagine the crowd sound as a loud 'CAW!' like crows cawing: caw -> カ, and the many mouths make the noise.