Japanese · Pronunciation Guide
su-mu, two short beats with two identical unrounded u's — never "soo-moo."
sumu
Two identical slots, high then low. Both vowels are the same, both beats are the same length, and there's nothing else in the word — which makes it an unusually pure test of whether you're producing Japanese u correctly.
Listen: 住む
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東京に住んでいます。
Toukyou ni sunde imasu.
I live in Tokyo.
Listen: 東京に住んでいます。
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東京に住んでいます turns it into 住んで, where the mu becomes a moraic nasal: su-n-de, three morae with a beat of humming in the middle. 東京 at the head is four morae with two long o's. The sentence therefore opens with held vowels and continues with a held nasal — different sounds, same principle.
"SOO-moo" — the sumo-wrestling pronunciation English speakers already know — is exactly wrong here on both vowels, which should be unrounded, and on the stress, which shouldn't exist. Spread your lips and check in a mirror: if they push forward at any point during this two-beat word, the vowel is English rather than Japanese.