Well, the thing is, that we cant hear it from here!!! So we have no way of knowing if the sound has deteriorated too much or not.

Go ask your local independant MoT testers opinion. Slip him a fiver for his trouble. If he thinks it too weak then go to the dealer and ask him to park yours next to a brand new A3 in the car park and have a technician sound both horns one after the other. Any significant difference should be easily audible "side by side". If the sound has reduced to below MoT standard (whatever that is) then I think you have a case.

As it's still under warranty you can reasonably ask for it to soj d the way it did when new. Most car horns (apart from those made by the Japanese, whose legendary politeness seems to require them to provide only an ineffective 'poop') are dual-tone, and I would guess that one of the 'tones' has gone off on yours.