Some horns have screws on the back that you use to adjust them with. If you can see it easily then try turning it up a bit if it has one.
It’s on the top of the radiator. I will have a little fiddle when I get home. With my bike that is.
if you can find the room for it’s depth, I recommend these: http://www.ebay.co.uk/itm/Stebel-TM80-Magnum-136dB-horn-Be-Safe-be-heard-12V-High-Tone-TM801-MAG12-500H-/120885494359?pt=AU_Motorcycle_Parts_Accessories&hash=item1c25564257#ht_1220wt_754
Might have water in it if it’s not sounding right or as above adjust the screw.
I have the low note one and they are brilliant, scares the sh*te out of cagers…
Got it from here… http://www.classicsagogo.co.uk/acatalog/Horns.html
I was going to send you some of my erotic literature, that would have got your horn alive and kicking again :w00t:
Another vote for them 
Haha certainly nothing wrong with that horn. Send it anyway! Is there pictures included? :w00t:
Another vote for the Stebel “low note”.
But it does need a relay. Another complication to a p*ss simple job.
Er no relay for mine, just a straight swap!
same here for my fazer 600 and er5
Bugg*r just realised you could have heard mine yesterday…and seen where it was located.
I have a few old horns off bikes that you can have for free and I help fit it aswel
Let us know