You say you have dual horns and they both got weaker at the same time?
If that is true, the problem is probably electrical like a weak connection or switch. That sounds logical but the reality is that one horn may have gone south and then the second went. It may not be easy to tell if both went at the same time or each one at a time slowly. As mentioned the horns don't last forever but if you want to check the electrical supply, unscrew the horns and pull the wires out until you see the crimp connectors. The horn bracket will be set in either adhesive or more likely silicone so you will need to work at it a bit. Once you have the crimp connectors exposed, use a voltmeter to measure the voltage between the connectors on the horn side when the horn button is pressed. It should read close to sitting battery voltage ( you don't need to start the motors as long as the batteries are decently charged.)
If the voltage is low and that would be a volt or more below standing battery voltage you have a wiring/connector/switch issue. Most likely the horns are bad. It is not super difficult to open the horns to check the vibrator contact but usually the inner guts are rusted and adjustment doesn't help.
I second Ongaro, and choose the stainless option.