I think there are 2 types of horns on mopeds. AC and DC horns, which are very different in operation. The AC horn, used on mopeds that connect to AC direct from the magneto, is nothing more than a buzzer. The frequency of the horn is based on the frequency of the magneto pulses. I think at high rpms the horn can't buzz as fast as the magneto pulses, that and the engine drowns it out. Hold down your horn button and slowly bring up the rpms, the AC horn should change tone with the rpm change.
If you have a regulator/rectifier in the horn circuit, or it is run from a battery, a DC horn must be used.
Some horns, Italian I think, have an identifier stamped on them, AC horns have "CA" stamped on them and DC have "CC". To tell what type of horn you have, hook it up to a battery of the correct voltage, the AC horn will make 1 click, but a DC horn will continue to sound.