Little Nightmares II

Towards the end of chapter two just after you rescue Six you will come to a point where you open a window and go across a plank to another window. In this room you will find a piano, simply run back and forth across the keys, the achievement will pop after around your fifth pass across the keys.

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.

Around the End of Chapter 2 you'll be met with a Piano after crashing it down jump or run across on the Keys for a given amount of time and the Achievement will Pop! shortly after

Video guide for Reference:

Pour débloquer le Succès/Trophée "Mono-tone" de Little Nightmares 2 qui est très simple, vous devez tout d'abord rejoindre l'école du Chapitre 02. Juste après avoir retrouver Six dans les toilettes, vous trouverez un piano suspendu dans une pièce. Pour obtenir ce Succès/Trophée, vous devez faire plusieurs aller-retours sur les touches du piano.

To unlock the "Mono tones" Achievement/Trophy in Little Nightmares 2 which is very simple, you must first join the school in Chapter 02. Right after you find Six in the bathroom, you will find a piano hanging in a room. To get this achievement/trophy, you have to go back and forth on the piano keys several times.