You can make all manner of musical instruments with an Arduino, from simple buzzers to creating MIDI interfaces and more, however standalone instruments are quite easy. This is demonstrated by Andrea Piombo & Silvio De Raeve who were tasked to build an interesting interactive device for a school project and created a neat piano with touch-sensitive keys.
The keys are crated by cutting aluminium tape into the shape of a small piano keyboard, which are then considered capactive touch sensors by the Arduin sketch. Sounds are generated with simple functions and sent to a small speaker. Andrea and Silvio demonstrate their piano in the following video:
If you're interested in detecting vibrations or working with making sounds and tunes for various reasons, we have a neat little SOUND: sound and buzzer module:
It can be used as a noise-maker driven by your microcontroller for audible feedback of events, and it can also be used as a knock-detector input to sense events and react to them. Includes a built-in 1M resistor to allow the piezo element to detect shocks. For more information and to order, please visit the product page here.