Hey hey hey
I recently got an iPhone 6S and was upset about the game being too quiet. It was my biggest complaint aside from the ads that is obviously everyones problem. I found out how to fix the volume issue; I thought it was the game but its the phone. Go to settings-general-accessibility and turn 3D Touch to OFF. Ta-Da! Problem solved, evenly loud music once again! Hope this helps somebody else who was having this issue and never thought about the 3D Touch feature.
KJ5wa about
Magic Piano: game by Smule