Korean · Pronunciation Guide
cheo-eum. Two unrounded vowels meet with nothing between them -- ㅓ then ㅡ -- and the ㅊ at the front needs a real puff.
cheoeum
What varies here is not loudness but the transition: two unrounded vowels in a row, differing mainly in tongue height, with no consonant to mark the boundary. Korean keeps the voice running straight through the join.
Listen: 처음
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오늘 처음 만났어요.
Oneul cheoeum mannasseoyo.
We met for the first time today.
Listen: 오늘 처음 만났어요.
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오늘 처음 만났어요 puts 처음 between an adverb and a verb, and 만났어요 gives you a geminate n followed by a resyllabified double s: man-na-sseo-yo. The soft vowel run of 처음 sits between two far more consonantal words.
Merging the two vowels into one long sound is the commonest outcome, and inserting a glottal stop between them is the other, and neither is Korean. Keep them distinct and connected. The ㅊ also needs its aspiration, since softening it gives 저음, a different word altogether.