The song is in Ab...just listen to the melody (soprano) and break down the chords into three part harmony (for alto and tenor)...I've heard the song done a number of different ways, some parts in unison, while other parts using harmony. short example of harmony break down off chords below, (I'm not at a board and going from memory, so this may not be totally correct).
On the turn-around at end of verse...Because My Worship, My Worship is for...
I believe soprano part is...
Be - Bb
cause - Bb
My - C
Wor - Db
- ship - Db
The Db soprano note is played with a Bb root bass (Bbm7 chord or 9 if you add the C), if you split the Bbm7 chord up with the melody (soprano note) on top it will look like below.
Bb/F-Ab-Db
Tenor will sing (F)
Alto will sing (Ab)
Soprano sing (Db)
Hope this helps a little.