Grumpy_Mike:
The way to go is to use elastic bands, and optical sensors.
You do not actually use the sound but use the data to trigger the appropriate MIDI messages.
How would you use an optical sensor with an elastic band? Where would it be mounted so it could view it?
How about a 2ft long capacitive touch slider? What sort of problems could I run into? I'm not sure how thin I could make it.