If the tempo of the song keeps accelerating or the key changes in every repeat, you have the Practice settings turned ON. Use the sliders to turn them OFF.
 
 
iOS/Android: Tap the ‘Practice’ button (graduation hat) in the Player controls;
 
Windows: The Practice settings are in the Player controls when on the right.  Use View > Toggle Player controls to show them on the right if they are at the bottom;
 
Mac: The Practice settings are in the Player Controls on the right sidebar (or bottom menu on older versions). If the Player Controls are minimised, then View menu/Maximise player controls, or click the Minimise/Maximise icon top far right.
 
 
iOS shown