Finally, a topic on which I might actually have some type of beneficial input! Did I say I love this board? Anyway...
My horns are Ongaro from the factory ('04) and I started having trouble within six months. One horn would sound the "wounded duck" call , then barely vibrated and I guess it was from the saltwater. My attempts to invade the horn itself did not bear fruit and, in a moment of frustration, I pumped a healthy dose of Boeshield T-9 into it (it just happened to be sitting nearby). Well, that killed it. Nothing, no sound or anything. I figured I would live off one horn for as long as it lasted, then do the replacement.
About three weeks later I was on the water and was goofing around with my guests when to my amazement both horns were working. Not only that, the formerly inoperable horn was as good as new! I immediately sprayed the other horn and the same phenomenon was experienced. First, no sound at all, then, like a firebird rising from the ashes, full bodied sonorous trumpeting a week later!
So, if your horn is not working, try the Boeshield spray cure. What have you got to lose? I merely spray them out with fresh water after every trip now and they are still blasting. That Boeshield stuff works great under the engine cowling too.