No confusion, I was talking about power LOSS in the wire.
Power lost in wire is a square function. If you mount two horns, replacing one horn, you have four times the load in the wiring!
As current increases the voltage drop (loss) also increases. power (watts) equals current squared, times resistance.
simply if you are running 1 amp with a voltage drop of 1 volt you will loose 1 watt. If you increase the current to 2 amps you will get a voltage drop (in the wire) of 2 volts now you are loosing 4 watts. This is why you must increase the wire gauge size. If you double the current you double the diameter of the wire which gives you 4 times the area......
You are correct that power does not necessarily correlate to loudness.