I’m co-teaching a course on polyphonic song in Hong Kong – the full title is long: 同流演藝課程：複調演唱：聲音訓練與劇場應用 Polyphonic Singing: Voice and Theatre Practice. Details in Chinese here.
Here come the details:
Polyphonic song is a tradition of singing together in different voices, which combine to make the whole sound. The origins vocal polyphony can be traced far back in the cultures of Greece, Albania, Serbia, and Georgia. Georgian polyphonic song is described by UNESCO as an Intangible Cultural Heritage of Humanity (as is Chinese tea!). Songs are taught through listening rather than by following scores, and are usually sung a cappella.