Me and my friends were still figuring out this new shooter. 20 seconds in, we were told we were on a quiet mission. Stealth was important. My friend was still testing what each key on his keyboard did.
23 seconds in, my friend found the hand grenade button.
This is how I handled the early Hitman games back in the day. Wasn't always obvious what you could use and simply testing the waters could fuck you up; but who cares when you can find a rocket launcher somewhere and just kill everyone? can't have any witnesses if no one is left alive.
Likely not to an appreciable degree. Horns make a sound louder by giving the thing producing the sound more air to push against, so the pressure waves actually build up and travel through the air instead of just pushing it around.
Meanwhile a gunshot is loud because it's already a massive pressure wave. Silencers work by containing the pressure spike and allowing the gas to expand and cool.
In fact, muzzle cones were added to rifles and machine guns during the early 20th century for a similar reason, to suppress the muzzle flash from the unburnt powder hitting fresh air and combusting by giving the gases a place to expand and cool. They did also tend to direct some of the concussion away from the shooter, but it's debatable how effective this was.