Synod of Laodicea, Can. xvi. 15:  “Besides the appointed singers, who mount the ambo and sing from the book, others shall not sing in the Church.”  Hefele thinks that this was not intended to forbid the laity to take any part in the Church music, but only to forbid those who were not cantors to take the lead.  See Bingham, Antiquities, III. c. 7; XIV. c. 1.