This Instructable (more of a basic mod or hack) can be priceless for new parents. Hope it helps you as much as it helped us!
Step 1: Locate the doorbell cover
usually found in close proximity of the actual door...near the ceiling. This one happens to be about 30 years old. Which reminds me: BUY NEW DOORBELL COVER.
Step 2: Remove cover
Simple enough. They aren't usually screwed to the wall or anything. The covers usually "hang" on the doorbell mechanism.
NOTICE: there are two flat metal "chimes" next to the tubes?
Step 3: Choose your magnets
Go to your nearest refrigerator. On it you will find upwards of 5000 magnets. At this point it is up to you which magnets you want to use. They must fit within the confines of the doorbell/cover, so choose wisely. (you'll see why in the next step.)
I choose pizza hut and A1 tattoos because who can resist a night out featuring pizza and piercings?
Step 4: Attach the magnets to the chimes
The actuator is the main workhorse of any doorbell. It has two electromagnetically controlled plungers that hit the chimes to produce the typical "ding-dong" chime so familiar to us all. One chime is the "ding" while the other is the.......uh......"dong". (why did I just think of Beavis and Butthead while typing this?)
The magnets should stick to the metal chimes and you want them to be placed so the plungers strike the magnets when you press the doorbell.
This makes the magnet deaden the sound of the chime to the point that you hear the ding-dong but the really is no sustain to the notes. It's more of a "did-dud" and it's very quick.
Step 5: Replace the cover
Put the cover back on and relax when your buddies come over and forget it's nap time!
When they ring the doorbell, you'll know it, but the baby won't wake up!