Pastor Danny takes us through the end of chapter 10:17-21 in this teaching. And in verse 16 Paul says "have they not heard", who is the "they" he is referring to?

He is talking about those who have chosen to disobey or disregard the gospel! And they [the Jews] chose to disobey not because they had not heard, but rather out of stubborn disobedience, clinging to the traditions of their fathers [forefathers] instead.