hum, sing softly, groan, moan
哼 centers on producing low, soft vocal sounds: humming a tune, singing gently, or making quiet groans and moans of discomfort.
哼 combines 口 (mouth) with 亨, which likely contributes the sound. The character expresses vocal sounds made with the mouth.
The mouth 口 beside 亨 suggests soft sounds coming from the mouth: humming, singing softly, or groaning.
For コウ, imagine humming a soft tune that sounds like 'ko-ou'—a gentle, drawn-out hum: ko-ou -> コウ.