What literary sound device/technique is expressed through the phrase "sudden departures", involving the articulation of the words? (alliteration, assonance, sibilance etc.)
The only "sound device" I can hear in that phrase is its meter, which is a catalectic dactylic dimeter. A dactyl is a set of three syllables, the first of which is stressed; in a dimeter pattern, the dactyl occurs twice: DA-da-da DA-da-da. "SUD-den de-PART-ure" fits that meter, except for missing the final, unstressed syllable; leaving off an unstressed syllable at the start or end of the metric pattern is "catalexis".
Well, there is no alliteration or assonance, and just a modicum of sibilance. Alliteration involves the beginnings of adjacent or closely connect words, assonance is repetition of vowel sounds that create an echo effect; and as sibilance refers to a hissing effect (usually achieved with multiple unvoiced s sounds — as in the word susurrus), and there is only one such sound in your example, I'm not sure what you're going for here? Perhaps if you described the quality you see in sudden departures that is noteworthy to you, we might be able to see what you're really asking.
I didn't think alliteration was completely restricted to the start of words. I think the repeating of the "plosive" "D" sounds Su*DD*en *D*eparture is a form of alliteration too, even though the "D" sounds aren't at the beginning of the word Sudden. This gives the phrase an almost explosive impact - which perhaps reflects the "Sudden-ness"?
Are you looking for "onomatopoeia", meaning a word that imitates the sound it represents?