When you edit a picture with the 3ds, select the effect to add stuff, like dots and such and select the music notes.
When you place one, it makes a sound, depending on where it is.
Unfortunately, the only notes available are C D E F G A B on 3 octaves.
There is no C# or such.

I didn't found any other video showing this so I might be the first one to discover it :D

Also, the grid used as image in the video is a grid I made myself based on where the sounds changed.