I bet this is possible. In other threads, I think we've discovered that the noisebox in the front of the car is simply a weatherproof speaker, so with a record board (similar to those used in birthday cards and such) outputting sound to the noisebox, you could probably set any sound effect you'd like. I can't think of the site off the top of my head, but I've found boards that you can load sound files and songs onto, and there are several variants of activation (light, motion, push-button, etc.).
After being around other EVs lately, I've been thinking again of unplugging my AVAS. Nobody outside the car hears it. I've come up right behind people in parking lots, and they don't know I'm there until the brake pump kicks in when I hit the brakes.
"Bear" - 2012 Diamond White Pearl ES with QC - 2/21/2013
"Koorz" - 2012 Cool Silver Metallic ES with QC - 1/5/2015
"Photon" - 2017 Bolt EV LT in Orange with QC - 7/31/2017
Solar-powered since 10/10/2013