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 by Smule