There was a recording of the horn in the Fremont test drive (or a week or two later, I forget). FYI.
Edit...
Here you go:
http://www.teslamotorsclub.com/showthread.php/9165-Horn-sound?p=156910&viewfull=1#post156910
Yup, Fremont.
But is that the same sound that comes out of a Production Sig?