make yourself a test switch, (12 volt on contacts) to test the horn in it's fitted location. Connect this to the horn, and see if it works. If it doesn't, then the ground or the horn are suspect - test the ground. If it does work, then the problem is your switch.
A softer or squeaky horn sound suggests that the contacts of your horn switch, or the connections in the wires to the horn, are corroded, making a bad contact. First check the wires and connections - if these are ok, dismantle the handlebar switch and clean the contacts.. or buy another horn switch.
I can think of nothing else that would give you that problem, but then I do not claim to be an expert.