This is Pedro Rubalcava‘s setting of the Entrance Antiphons of the four Sundays in Advent. He adds a fifth to seventh stanza with general Advent themes, so you could sing the whole song or pick the appropriate verse for the week in Advent.
The text is here (page 5) and the sheet music can be purchased for download at OCP, where you can listen to a snippet of the original.
My backing is just for using the song as an entrance: refrain/verse/refrain, rather than the whole thing, but it gives you the idea of the 4/4 refrain moving into the 6/8 verse.