What are the requirements for a car horn(button) to pass inspection?
Does it have to operate (activate) as originally designed, or will a modified or repaired horn button pass inspection as long as it is accessible by the driver?
This may vary from state to state (or place to place, depending on who regulates inspections where you’re at). It may also vary depending on who’s doing the inspection: more easy-going inspectors may only care that there a) is a horn, b) it works, and c) reaching it does not present a possible hazard to the driver or passengers, while someone who’s more by the book may be a bit more anal about it.
Speaking from experience, as long as a) there is a horn, b) it works properly, and c) it can be used safely, it’ll pass. (Back in '94, we had a behemoth of a van that the horn had shorted out on. Long story short, we ended up having to rewire the horn to the cruise control button on the turn-signal indicator lever. Took that van in two-three months later in for the yearly saftey inspection, and it passed. [sub]Barely though, but that was because of the shape the body was in, not because of the horn.[/sub])
<< Yeeee-haaa! >>
I wired a musical air horn up to my standard horn button and disconnected my stock horns. Never had a problem. 'cept when I took it in for an oil change, the guys got a nice surprise when they tested it. 
I don’t know where you live, but according to the Texas Department of Public Safety:
“Every motor vehicle shall be equipped with a horn (electric or air) in good working order and capable of emitting a sound audible for a distance of 200 feet or more, but no horn shall emit an unreasonably loud or harsh sound or a whistle.”
Some places may not test or even care about the horn, especially if your car is a late model or in good condition. I know the place down the street from me is pretty lenient about those kinds of things.
In Missouri, it does not have to work through the original switch. A dash-mounted horn switch is acceptable, so long as it functions properly.
OOOOOoooooooooooooogaaaaaaaaaaaaaaaaa.
Heck, FoMoCo used to mount the darn horn button on the end of the turn signal lever, and that was legal.
A button or switch within the drivers reach seems to be the key. As for the actual sound, sirens and whistles are reserved for certain vehicles, so they could flag your car there.
But, when it comes to OOooogas, musical style horns, truck style horns and such, there must be some subjectiveness to it.
The real issue is, if them thar Duke boys never gots written up in Hazzard county for their musical horn, I’z might be guessin’ almost anything goes…
…and is it my imagination or did their horn play “La Cuckaracha”
???
Dukes: I know…it was Dixie.
Triumph did this on the TR-7 and TR-8 starting in 1975 and continuing until their demise in 1982. Took quite a bit of getting used to. Not to mention beeping your horn everytime you tried to turn left!
For a time I was driving a Ford Escort which had perpetual horn problems. It didn’t pass inspection with a horn which just made loud clicking noises when the center of the wheel was pushed, but it did pass inspection (this was in Wisconsin, btw) with a handheld air horn that was mounted under the hood and triggered by pulling on a string which went through the center console and came out through one of the air vents.
Later it also passed with a boat horn mounted where the air horn had been, which was triggered by a big red light-up button which read “Panic!” mounted in the middle of the steering wheel.
However, a cop who pulled me over wrote me up for that, as well as a plastic snail (Escort->Escargot Ha!) glued to the dashboard as a “visibilty hazard”, but she was just being a jerk and later the judge dismissed the whole thing.