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.)
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I wonder if people would be more apt to honk their horns if it played the latest Pussycat Dolls or Jessica Simpson jingle? And if that were the case, is this really an advancement in technology that we as a society want?