Note: You do not need listen or sing for this quest
In Makon, The Fifth Heaven you should LISTEN in each room, and occasionally you will receive a message saying that you have heard the notes of the Trisagion in your head, along with a clue to the location you must sing them at.
The words are SANCTUS DEUS, SANCTUS FORTIS, SANCTIS IMMORTALIS but you must have heard them before you can use them.
The notes seem to come from a place with yellow bricks! '- The Yellow Brick Road' (WSWWS from Choir)
The notes seem to come from a place where a rhapsody echoes through the Heavens! - 'Caught in a Landslide, No Escape from Reality' (EESE from Choir)
The notes seem to come from a place by the river of dreams! - 'The River of Dreams' (ENE from Choir)
Simply go to the appropriate room and SING (the words you have learned), when you sing them you will be transported to the room above the 'The Song-Uttering Choir' and you will be faced with an aspect of Samael, these are relatively tough, defeat it and move down to return to the choir. Now continue searching for the next words and repeat until you have sung all three pieces and defeated each aspect of Samael.