French · Pronunciation Guide
duh-VWAHR — the first syllable is a weak schwa, the second carries everything.
de-VOIR, with a heavily reduced first syllable and the full weight on the second. Of the -oir verbs this one has the weakest opening, because the "e" is a schwa rather than a full vowel.
Listen: devoir
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Je dois partir.
I have to leave.
Listen: Je dois partir.
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"Je dois partir" uses the conjugated "dois" /dwa/, a single syllable where the "oi" is again "wa" but the "s" is silent. So the infinitive has two syllables and the conjugated form has one. "Partir" is an -ir infinitive, and unlike -er verbs its final "r" is pronounced: par-TEER.
Learners give the first syllable a full vowel, saying "day-VWAR", which makes the word top-heavy. It should be a barely-there "duh". The other habit is treating "-oir" as two syllables. And a semantic note worth having: as a noun, "les devoirs" means homework, from the same root — the verb means "to owe" as much as "must".