dumb, mute, unable speak
瘖 centers on the inability to produce speech: being mute or dumb, unable to speak.
瘖 combines 疒 (sickness) with 音 (sound), suggesting a condition related to the loss of sound or voice. The exact historical development is uncertain, but the phono-semantic structure is plausible.
The sickness enclosure 疒 wraps around 音 (sound). Picture an illness that traps sound inside, leaving the person mute and unable to speak.
For イン, imagine an inn where silence is enforced: inn -> イン, and the mute person inside the sickness enclosure cannot make a sound.