Time for me to start correcting some of my own manuscripts and choir notes, otherwise someone might catch on and think that we were instructed to sing like goats. (LOL)
I for one always thought "a cappella" was supposed to be spelled "a capella". But, according to this editorial from The Catholic Choirmaster (a 1933 issue edited by Nicola A. Montani), here's the difference:
If one is considering a soloist, either male or female, who with a theatrical "tremolo" wishes to give greater expression(?) to his or her performance, even accompanied by a most capable orchestra, one must perforce write "a capella," which translated from the Latin means "rendered by a goat," or "in the manner of a goat."
But if one is treating of a choral body interpreting a beautiful piece of music in the Palestrinian style without accompaniment (even of the organ), it is necessary to write "A Cappella" because historically this term was applied to those compositions rendered by choristers vested in the "Cappa" or singer's cape.
Hat tip to Musica Sacra