Gagok
Gagok is a genre of  Korean vocal music for mixed female and male voices.  
Accompaniments and interludes are played by a small ensemble of traditional Korean musical instruments. 
It is inscribed in UNESCO Intangible Cultural Heritage List from 2010 and enlisted as South Korean Intangible Cultural Property from 1969.

External links
This site features an image of a kagok ensemble.
http://www.asianinfo.org/asianinfo/korea/perform/distinguishing_features_of_korea.htm
Coralie Rockwell: kagok a traditional Korean vocal form