Hey, why not!
You can personalize the ring of your mobile phone, so why not personalize the honk of your car's horn? 
(That would be cool. )
(And why not call them "horntones" with a respectful nod to "ringtones.")
Put this one in the "why didn't I think of that" category and check out the details here.
Reminds me of the days when my friend's older brother had a megaphone system in his car so he could tell other drivers to move over (or worse) as he motored down various highways and byways -- yes, the Hart family ...
And while we're at it, check out the Car Horn Organ here.
(That would be cool, too.)
