咦

Left mouth radical — the indexing element. 咦 is an interjection of surprise ('huh? what?'), so the mouth anchor is literal: a sound coming out involuntarily. Puts 咦 in the voice-particle family with 哎 (oh), 哦 (oh I see), 嘿 (hey), 嗯 (mm) — a whole shelf of small mouth-prefixed exclamations marking different shades of reaction.

Right 夷 supplies the sound — yí matches the parent reading with no drift. 夷 originally depicted a person with a great bow (大 + 弓), referring to the eastern peoples; later it took the senses of 'level, calm' and 'foreigner.' For 咦 it functions purely as a sound-marker, lending the reading to this expression of mild astonishment.