Long ago----when I was six------we had to repeat "THE LORD's PRAYER" in school---
I had the misfortune of being situated next to a kid who---EVERY MORNING---
instead of "hallowed be thy name" said "HAROLD BE THY NAME" I never got
over it
It is based on the Gospel of Luke 21:
8 And there were shepherds living out in the fields nearby, keeping watch over their flocks at night. 9 An angel of the Lord appeared to them, and the glory of the Lord shone around them, and they were terrified. 10 But the angel said to them, “Do not be afraid. I bring you good news that will cause great joy for all the people. 11 Today in the town of David a Savior has been born to you; he is the Messiah, the Lord. 12 This will be a sign to you: You will find a baby wrapped in cloths and lying in a manger.”
13 Suddenly a great company of the heavenly host appeared with the angel, praising God and saying,
14 “Glory to God in the highest heaven,
and on earth peace to those on whom his favor rests.”
15 When the angels had left them and gone into heaven, the shepherds said to one another, “Let’s go to Bethlehem and see this thing that has happened, which the Lord has told us about.” . . .
Nowhere in Scriptures is there any reference to the angels singing. But music being such an important component of human culture for millenia, it was only natural for the hymnists to translate 'saying' into 'singing'. I put that into the category of if it isn't true, it ought to be.

But yes, the angels were the heralds to the shepherds to tell them that the Savior of the world is born.