This usually gets filed away in the “Ancient Greek rhetorical” education requirements, that I often simulate and emulate in order to run experiments comparing ancient and modern teaching methods. With sound recording tools, one can even make a choir out of a single person even.
Rhetorical skills were essentially speech and charisma abilities talented individuals trained in order to have political and social influence. In peace, that would be so, in war it would be used for slightly different purposes such as improving unit morale. Singing allows modulation and control of tones, which is borderline between rhetorical skills and hypnosis skills. The breathing practice also improves internal martial arts applications and dexterity skills.