Actors and actresses performing cheo — whether as central characters, heroes, heroines, buffoons, or villains (often a person who counters society's moral standards, not necessarily one who does evil) - must sing clearly to show their characters' unique physical characteristics.
They must enunciate both the tones and words. Each feeling - whether happy and joyful or sad and miserable - is expressed with a certain unique nuance of voice: metal voice (kim), earth voice (tho'), copper voice (dung), and so on. These sung sounds give rise to sophisticated onomatopoeic associations that audiences treasure.