The Arduino platform and digital music works very well, and a xylophone project by Instructables user 'audreyobscura' is a great example of this. By using an Arduino Mega board for the extra analogue inputs, a dozen piezo sensors and some laser-cut parts she has created a xylophone that can appear as a music instrument in various software packages such as "Garage Band" in MacOS. The Arduino measures the signal from the piezo sensors which are actuated when a beam is played, and sends the signal back to the PC via USB. By using a serial>MIDI software the xylophone then becomes a full MIDI-interfaced instrument. In the following video you can see the whole process in action:
So if you're interested in the Xylophone, need more analogue inputs or have a project that's outgrown your Arduino Uno-style board, move up to the Freetronics EtherMega:
Quite simple the EtherMega is the fully-loaded Arduino-compatible board on the market today. Apart from being completely Arduino Mega2560-compatible, it includes full Ethernet interface, a microSD card socket, full USB interface, optional Power-over-Ethernet support and still has a circuit prototyping area with extra I2C interface pins. So if your project is breaking the limits, upgrade to the EtherMega today.