Out of curiosity, was there not a time in Church history when music was absolutely forbidden during Mass?
Not as far as I know. Gregorian chant goes back more than a thousand years and before that there was Ambrosian chant. And monks have always sung the offices, I think. I can't see why singing at mass would have been prohibited. Of course at the Reformation all sorts of things were banned or came under suspicion.
Luther himself, however, was a great believer in getting the congregation to sing, hence all those chorales that people like Bach later set with such beauty and ingenuity.
The one below was composed by Johann Schop in the c.17th and harmonised a century later by Bach in the Christmas Oratorio as follows (also known in English as "Break Forth O beauteous Heavenly Light") :
As ever with Bach, the bass line is to die for......