The tone screw on the horn can be used to vary the sound of the horn , but will not change the volume. Exposed electric horns can rust internaly, so you may be looking at replacement.
Have you tested, power and ground, at the horn ?
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.