Pronunciation: Like the two words "plan" and "gent".
Origin: According to the Online Etymology Dictionary, "plangent" comes from Latin plangens meaning "to strike or beat".
Why use it? Not only does plangent not seem to have any real single-word synonyms, but I think it can be used in many more figurative ways. Disaffected voters and their spokespeople often sound plangent in a figurative way, even when speaking in a normal tone of voice. And many slow-moving tragedies have a plangency -we can see them happening, but there is nothing we can do from a distance.
Plangent was used by Alice Walker in "By the light of my father's smile" in this passage:
She moans along with the woman who is singing -- wailing, really -- her hands gripping the steering wheel to the plangent cries of the singer and the sobbing of violins.
Sources: In addition to those cited in the text: Wordnik