Question about a famous saying?maybe from bible? (not sure)?

Folks are right that the word is "herald" meaning messenger. But the words do NOT mean "listen to the angels," because the entire line, from the famous hymn by Charles Wesley, is "Hark! The herald angels sing glory to the newborn king." This comes from the Gospel, "And suddenly there was with the angel a multitude of the heavenly host, praising God and saying "Glory to God in the highest and peace on earth goodwill towards men." So, what it means is "Listen! The angels are singing glory to the newborn king."