Learn Turkish: Kim susmuyor? - Who is not being quiet?

The root verb is susmak (to be quiet), so the stem is sus-. Usually, the negative suffix is -ma or -me. However, when we add the present continuous suffix -yor, the vowel in the negative suffix changes (narrows) to match the root. Because sus- has an u, the negative suffix becomes -mu, giving us susmuyor (is not being quiet).

Why isn't there a personal ending at the end of susmuyor?

In Turkish, when the question word kim (who) is the subject of the sentence, it is treated like a third-person singular pronoun (he/she/it). Since the "he/she/it" form doesn't take a personal ending in the present continuous tense, we just leave the verb as susmuyor.

I've heard people just say "Sus!" before. Is that the same word?

Yes! Sus is the direct command (imperative) form of the verb susmak. By itself, it means "Be quiet!" or "Hush!". In this sentence, we are taking that exact same root and adding the negative present continuous suffixes to ask who is not doing it.

Could I translate this as "Who is not speaking?"