looks to me like the tab would direct the sound in a forward direction, towards the intended imbecile ... if thats true, then removing the tab would cause the sound to travel downward, making it seem louder inside the car.
ie: i would think that pointing the horn towards one self, rather than away, would make the horn seem louder.
FWIW: In SoFLA everybody has the windows up and ac on; gomers* have hearing aids off and kids have tunes blaring (or subs buzzing so bad, you can hear their license plates rattle). If they only realized how moronic they look with sideways hats and buzzing little go carts. You really need a beatup 76 blazer and let em pull out...... horn not even needed.
*Gomer= Get Outof My Emerg Room. Typically a very old chronically sick nasty old fossil that is very ill and is ready to die..