The lyric from “Amazing Grace” is “How sweet the sound,” so the missing word is sound.
The line opens the hymn’s famous first stanza: “Amazing grace! How sweet the sound / That saved a wretch like me!” The words were written by English clergyman John Newton and first appeared in print in 1779 in Olney Hymns, a collection produced with poet William Cowper.
Newton’s text grew out of themes of spiritual conversion, forgiveness, and redemption. He had worked in the Atlantic slave trade before becoming an Anglican clergyman and later an outspoken abolitionist. The hymn’s opening phrase is not describing a particular musical performance; “sound” refers poetically to the message of grace itself.
The melody most listeners associate with the hymn is “New Britain,” a traditional American tune that William Walker paired with Newton’s words in 1835. The text and melody therefore have different histories, another reason the song is sometimes discussed as both an English hymn and an American musical tradition.