Korean · Pronunciation Guide
ah-nee-yo, three light beats. Every vowel is pure and short, and none of them takes an English-style stress bump.
aniyo
Three even syllables on a gently falling contour, the natural shape of a soft refusal. Note also that the older written form 아니오 and the spoken 아니요 have converged for most speakers, and the pronunciation you want is the one the romanization gives.
Listen: 아니요
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아니요, 몰라요.
Aniyo, mollayo.
No, I don't know.
Listen: 아니요, 몰라요.
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아니요, 몰라요 pairs it with 몰라요, where the ㄹ of 몰 and the ㄹ of 라 combine into a long, held double l -- mol-la-yo -- rather than two separate taps. That geminate ㄹ is one of very few genuinely long consonants in Korean.
English speakers stress the middle syllable, ah-NEE-yo, which turns a mild 'no' into something that sounds irritated. Keep all three level. There is also a temptation to rush it into 'ah-nyo'; that contraction does exist casually, but it sounds curt in the polite register this word belongs to.