Candace Whitcomb's organ playing and loud singing create a disruption in the church service, and they affect the story by drawing the attention of the congregation away from Alma Way's solo.
What does the organ playing do ?
Candace Whitcomb's organ playing and loud singing create "a disruption in the church service" because they interrupt the ongoing hymn, causing the congregation to turn their attention away from Alma Way, who was singing a solo at that moment.
The sudden appearance of another female voice singing a different hymn during the church service would likely be considered disruptive and unexpected, as it deviates from the planned order of worship.