my experience with that
i got very mad at that friggin noise once, on a 2001 v6. you are correct; it's like a little speaker on the circuit board. this one was down next to your left foot as a driver, fortunately i guess. well anyway, if you fill the little hole on that speaker with rtv and put it all back together, you get very very very light chime; you actually have to stop and listen for it.. if you are getting in and out you'll never hear it.
it worked for seat belt noises, lights-still-on noises, and key-still-in noises
i miss it dearly on my s197. yes i looked it up and found the instrument cluster makes the noise.
i had the instrument cluster just a few screws from out just a month ago; i had to take the plastic steering column loose to tighten the steering wheel adjustment thing.. it's not really difficult. i think i remember starting around the headlight switch with all the plastic stuff. a few screws.
i'll eventually "fix" this one too; it was coming up soon. i cannot stand auto locking doors, re-locking doors, automatic lights, beeps for dumb things. i think once in my life, on my 05, i did hear a beep and determined the trunk was popped, and flopping a bit, but there was a corresponding light in the dash, so the beep really wasn't needed. i'll photo it with my phone when it happens, if you havent done it.