In regards to the second verse: I think that it might be a reference to a few of the original lyrics from Amazing Grace: "Amazing grace / how sweet the sound / that saved a wretch like me". In this case, that "sweet sound" of grace is "like razors" in the listener's ears. He hears the sounds of redemption, but because he is so wedded to his vice, the thought of being redeemed is painful to him.
In regards to the second verse: I think that it might be a reference to a few of the original lyrics from Amazing Grace: "Amazing grace / how sweet the sound / that saved a wretch like me". In this case, that "sweet sound" of grace is "like razors" in the listener's ears. He hears the sounds of redemption, but because he is so wedded to his vice, the thought of being redeemed is painful to him.