This is a project that a person did with Arduino and Parallax PING sonar sensor. It is a special instrument that can make sound without touching it. Therefore, when people make a certain amount of distance between the sonar sensor, it will come up to some different tones. As the vedio displays the “Jingle Bell song”, people can also make their own songs~!

P.S. Here is the link for more information about this person’s Arduino Theremin.
http://alandtech.blogspot.com/2007/12/arduino-theremin.html