Gagaku, the "Music of a Thousand Autumns," is supposed to be the oldest unchanged musical tradition in the world, going back to the eighth century or so. I have doubts about that "unchanged" bit - all musics that are passed down orally change over time. I am convinced that the moments when the ryuteki flute and the oboe-like hichiriki diverge melodically are due to accidental misrememberings which became permanent.
Those two melodic instruments are accompanied by the sho, a kind of mouth organ capable of playing several notes at once, the koto and biwa (string instruments), and several types of drums. The music is strange, slow-moving, and haunting. I was lucky enough to hear three gagaku performances (or rather, two performances and a rehearsal) on a trip to Kyoto several years ago. I fell in love with the music, which in an odd way always reminds of the blues - probably due to the frequent use of bent notes.
Everest issued this LP by the Nippon Gagaku Kai in 1972, but I suspect the recordings were made in the 1960s. Here's a typical piece, "Etenraku," performed in the Hyojo mode.