To unlock the Mono Tones achievement in Little Nightmares 2 you'll need to run across the piano keys on the piano that can be found just before leaving the school (chapter 2) this is the piano that you have to jump on with 6 to push it through the floor. Here is my own video guide that shows how to get the Mono Tones achievement along with all of the other collectibles/achievements that Little Nightmares 2 has to offer. The Mono Tones achievement guide is shown at around the 11:05 mark during the video.