Interesting but erroneous William Williams, Pantycelyn was NOT known as the "Sweet singer" because he was a good singer.....he got this nickname because of his hymnwriting gift.
The hymn was written in Welsh.
"Guide Me, O Thou Great Jehovah" was written by William Williams (1717-1791), translated into English by Peter Williams (1722-1796) and others, and composed by John Hughes (1873-1932) who created the tune "Cwm Rhondda" for the hymn.
The Welsh audio clip at the beginning of your talk is the tune "Cwm Rhondda" but the words are of another hymn by Ann Griffiths who was a godly young hymn writer who died when she was 38.
The theology of her hymns and those of William Williams, Pantycelyn are theologically sound.