MIDI

MIDI (Musical Instruments Digital Interface) was invented to connect keyboards to external sound generators. MIDI transports commands ("MIDI messages") to start and stope notes, to change the volume etc. 16 "channels" (roughly comparable to the manuals of an organ) can be handled independently.

MIDI is a cheap way to get up to 88 keys with velocity sensing and global pressure sensing ("channel aftertouch"), possibly even per-key pressure sensing ("polyphonic aftertouch"). On top of that, very inexpensive controllers with motor faders and LED-illuminated rotary encoders are available. You'll find MIDIfications of drums, monochords as well as clarinets. The possibilites are endless. Doepfer's Theremin controller requires an analog-to-digital converter, however.

The C# MIDI Toolkit allows you to send and receive MIDI data with as few as five lines of code.

Sonification

Sonification is to sound what visualization is to graphics. The major conferences: ICAD 2005, AGS 2005, ICAD 2006, ICAD 2007.

Etc.

Multitouch sensing through LED displays. This should work with the Arduino, too, if you use a demultiplexer to control the vast number of LEDs and use an analog (de-)multiplexer to read the voltages at the LEDs. According to its specs, the Arduino's analog-to-digital converter would be fast enough for that.