The 'R' key is a developer's shortcut. I forgot to turn it off in the latest update, sorry bout that! The music layering itself is also because of the shortcut key. Select Restart from the pause menu for a proper reset and the music should go back to normal.
Thanks for pointing it out! I've just uploaded a new version, the 'R' key should be disabled now. Please do not hesitate to comment again if you have more feedback :)