Hey joe, good choice of horn!
My old Ford escort has the Ford optional extra duel tone horns fitted! ;)
if your a3 is like the previous model a3 and the model after yours, then you should find the horns (low tone and high tone) either behind the driver or passenger side fog light cover :)
couple of pics for reference on other a3 models. Couldn't find any pics for your model though. Sorry.
cheers
steve