Possible Clues:

• Is a farewell able to be sung?
• One musical work you can say good-bye to
• Choral work can go on so long
• In a sign of gratitude unable so to be set to music?
• Vocal work, maybe by Bach
• Bach's 'Pilgrimage,' for one
• One of 200+ by Bach
• Sacred choral piece
• Is an expression of gratitude unable to be sung? (7)
• Is one able to say farewell to this, by the sound of it?