French · Pronunciation Guide
loh-TON — the "au" is a pure "oh", the "m" is silent, and the "n" is real.
l'au-TOMNE, weight on the second syllable. The silent "m" is genuinely odd and worth flagging: it is a Latin fossil from "autumnus", kept in the spelling and dropped in speech.
Listen: l'automne (m.)
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En automne, les feuilles tombent.
In the fall, the leaves fall.
Listen: En automne, les feuilles tombent.
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"En automne, les feuilles tombent" links "en" into the vowel: /ɑ̃.n‿o.tɔn/, with the nasal releasing into a real "n". "Feuilles" has the "-euill-" spelling giving a rounded vowel plus a "y" glide: FUH-y. "Tombent" has a silent "-ent", so it is one syllable ending in a "b": TOHNB.
Learners pronounce the "m", producing "oh-TOM-nuh" with three syllables. The "m" is silent and the word is two beats. They also nasalize the second vowel by analogy with "nom" — but the following "ne" keeps everything oral. English "autumn" also silences its "n", which is the mirror image of what French does.