In almost all words in English whose spelling ends in -mb, the b is not actually pronounced. So all these words are pronounced without the b: bomb (rhymes with from), lamb (rhymes with ham), thumb and dumb (rhyme with gum), climb (rhymes with time), womb and tomb (rhyme with room), and limb (rhymes with Jim).
And how do you pronounce -mb- in the middle of a word? That depends on whether the word is related to one whose spelling ends in -mb. If it is, then the b is not pronounced: so you won’t hear it in bomber, lambing, climbing, dumbest, etc. If there is no such related word, then the b is pronounced, so timber, lumber, clamber, amber, etc., are pronounced the way they look!
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