SoMeThInG iS WrOnG wItH yOuR sHiFt KeY oR tHe "InTeRfAcE" iS mEsSeD uP.
pLeAsE cHeCk It.

Anyway, it's pretty hard, and not really advisable to kill the chime - after
all, it only goes off if the door's open or your seatbelt is unbuckled, and
the key is in the ignition. But, it is really loud. Several here have found,
and taped over the opening in the chime. It's still there, but it reduces
the level below the level at which OSHA requires earplugs.

First - find your shift key.

Your dealer was not lying to you - they can't disable any safety features.
You can, but they can't.  You're right - it is loud and annoying.  Your best
bet is to either tape something over the chime to lower its output, or to
pull the wires.  I can't tell you where the chime is located because I've
never searched for it on our Sonata.  I may at some point, but right now my
wife drives it more than I and as long as she does not complain, I'm not
going to create any more "projects" for me to worry about.  You should be
able to track it down though.  It may take some crawling around but I have
to believe you can find it in there.

WHY ARE YOU SHOUTING??? DID THE CHIME MAKE YOU DEAF???

THE EASIEST WAY TO AVOID HEARING THE CHIME IS TO WEAR YOUR SEATBELT!