Japanese · Pronunciation Guide
ko-e, two beats — the o and e are separate morae, with no glide between them.
koe
Two beats, the second of them a bare vowel. Standard speech puts the pitch high on the first and drops it, but the timing is what English speakers get wrong — the e must be long enough to register as a separate mora rather than as the tail of a diphthong.
Listen: 声
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彼女は声がきれいです。
Kanojo wa koe ga kirei desu.
She has a beautiful voice.
Listen: 彼女は声がきれいです。
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彼女は声がきれいです ends with きれい, whose final "ei" merges into a long e — so the sentence has two adjacent vowels that must stay apart in 声 and two that merge into one long vowel in きれい. Opposite treatments, and both are correct.
"KOH-way" glides the first vowel and inserts a w before the second. There's nothing between them. Freeze the o, then start the e cleanly with no consonant or glide. A tiny deliberate break helps at first, and can be smoothed away once the two-beat count is secure.