qw3rky made a simple music playing application called “Toy Piano DS”. You play a rainbow-colored mini piano keyboard on the bottom screen and all the notes that you play show up on the top screen on a musical staff. The “piano” sound is nothing fancy. It just comes from the procedural sound generators (PSG) on the DS, which makes for an odd combination of 8-bit retro fuzz and Fisher-Price aesthetics.

Controls:

Stylus – play keyboard
Up – raise pitch by one octave
Down – lower pitch by one octave

http://forum.gbadev.org/viewtopic.php?t=15195