dumb, mute, become hoarse
啞 centers on the inability to produce normal speech: being mute, dumb, or losing one's voice to the point of hoarseness.
啞 combines 口 (mouth) with 亞, which likely contributes the sound and may have originally depicted a person with a speech impediment. The exact historical development is uncertain.
The mouth radical 口 beside the complex shape 亞 suggests a mouth that cannot form words properly. Picture a mouth blocked or tangled, unable to speak clearly.
For ア, imagine a person trying to speak but only managing a weak 'ah' sound: ah -> ア, the sound of a mute voice.