'Let me sing you a psalm... eh, too much trouble. Let's find something more fun to do.'
As No. X Bell Ringer, Acappella can control the 'Air' and hear even the faintest sounds around her, including human heartbeats.
Perhaps it's because the world is too noisy that Acappella chose to join the Choir, using music to drown out the sorrowful sounds in her heart.