What is needed here is to source a accellometer/vibration sensor with potentiometer characteristics -- the accellometer/vibration sensor would read in degrees thus regulating voltage.

I did a little google-ing about and there look to be components that do this however I've yet to locate specific part numbers -- but then I only looked for a few minutes.

I've been toying with a similar idea to yours -- my thought was rather than modifying the digital sample (sound effect) digitally and playing it, the device would modify the playing sound in an analog fashion. Think of this kind of like effects on an electric guitar -- although the source is originally digital in our case. I suppose you could also think of it like the "Loudness" setting on a lot of audio components and/or bass, treble, volume adjustments.

These are all generally analog modifications to the analog signal before it passes into the amplifier -- although volume controls the amp specifically.

There are a lot of examples of PIC sound players on the web to play samples this would need to be mated with a micro-controller, sensors, and amp. An ASIC to read sound samples off a memory card would be nice too.