Well I tried my horn the other day and the magnum was a dull neerrrrr! sounds rather than the "ear splitting" ( account by Blue Bell Crewe bike tech who stupidly pressed the button whilst looking under the beak

) sound I was used too.
So whipped it off fairly sure water had got in.. and yepp.. turned it a few times along the line of the spiral and out came alot of water. Stuck some tissue paper in it and left it to dry out. Refitted and its back to its ear damaging self.
So if your horn isnt new, try turning it along the sprial axis.. otherwise if its brand new take it back and get another, might just be a dodgey one sadly.