In order to unlock this achievement you must have discovered the brick build "Giant Piano Keyboard" commonly found in large chests but could also be given as a reward from a minifigure.
Once discovered, unlock and then place the build on a surface and hit every note on the keyboard. Simply running across the keyboard will cause your character to miss several notes - it helps to have one of the tools out (landscape, discovery) so that you walk slower. You should be able to cross the entire piano several times in 10 seconds. Make a pass on both the white and the black keys.
Thanks to Turnedmonkey for the assistance.
Edit: M0j0s d0j0 has commented below that you can also use the paint tool to paint some landscape or bricks using the "notes" paint once discovered in the desert area. If the piano build doesn't work for you this may be another way to unlock the achievement.