Try replacing the horn with an aftermarket horn. They've probably got 8 or 10 different ones at your local Auto Zone. All aftermarket horns have the sp listed on the back. Your stock horn is probably about 90dB. Around 60dB should be enough to reassure you without waking the neighbors. If you can't find a horn quiet enough, you can get an air horn with a plastic trumpet, and drill some small holes near the base. This will decrease your volume, but it will also change the pitch. As long as you keep it in the key of F-Major, no one will think it's out of place (this is the key of all North American automobile horns).
Down side is, your horn won't be very loud, but how often do you really use it anyway?
And lastly, if all this sounds like too much trouble, goto the junkyard and get one off of a 2nd generation Elantra, the wires should match up, and I'm pretty sure those old Elantra's hold the world record for the weakest horn ever.
Good luck.