9 Pliny testifies that the same thing held true after apostolic times. He writes that apostates from the Faith said that "this was their greatest fault or error, that they were accustomed to gather before dawn on a certain day and sing a hymn to Christ as if He were God." These words of the Roman proconsul in Bithynia show very clearly that the sound of church singing was not completely silenced even in times of persecution.
Source: Musicae Sacrae (Vatican.va)