I just took the two 6 volt horns off of my '46 project. I am in the process of re-wiring everything and changing to a 12 volt system. The horns worked fine with the old 6 volt battery but when I tried them with the 12 volt source they wouldn't work. It was a battery charger with an 8 amp limit so I am thinking that the limit on the current is the problem and they might work with a 12 volt battery. I say all this to recommend that you make sure the wiring isn't limiting the current on your horns. If you can isolate one horn and use a jumper wire from the battery to the horn terminal (almost ALL horns ground by mounting them) that would tell you for sure. It should sound if you have a wiring problem, or a ground problem. If it doesn't sound then the horn itself might be just too old. I think they used some kind of polymer or plastic for the diaphrams on those horns and it might be old and brittle. I found this site if you want to have them rebuilt.
http://www.classiccarhorns.com/