This project is fairly far down my to-do list, but I'm planning to incorporate it into my build of the CANary project. Reprogramming the chip on the stock board can be ruled out, since if we could even get a blank part it would be impossible to reverse-engineer if the code protection bits are set, might interfere with other functions of the module, and anyway would not provide a user-configurable solution.
I opened up the VSP box and found the pinout marked clearly on the PCB. There are actually two speakers, back and front, both 4-ohm types, coming directly back to the VSP box. My preferred plan is to insert a new circuit inline with the cable going to the VSP box and use a relay to switch the speaker to a new audio amplifier. Generated sound will come from the mbed microprocessor in the CANary module. Then we have full control over the software, can play WAV and MP3 files off an SD card, select the sound on the fly, and even link it to CAN bus events like gear shifts and vehicle speed.
If anyone with the skills to do this gets to it before me, I'd love to see how it goes and help out. It'll be July at the earliest before I get to it. Still need to assembly my JuiceBox, fix my OpenEVSE, and, oh yeah, finish graduate school.