You are correct, there is a device that looks like a speaker in the front right side of the i.
I am NOT suggesting this be done for safety reasons so do it at your own risk but.....
You would start by taking off the top/inner plastic liner in the right front wheel well under the car.
The speaker is located there facing the right tire.
There is a push on clip type wire connector that when removed will disable the sound. It's a bit tricky to get unclipped but it does come off.
Put electrical tape on the two mating connections to keep water and dirt out.
New to this forum. This is old thread, but I am doing this tomorrow! I hate the sound, it is not loud enough anyway. I looked for a fuse, but did not find, so will take the wires off as you suggested. I like QUIET.