it's in pvk/sound/hd_horn.wav, not horn.wav that's perhaps why you didn't see it
I think he doesn't have the sound file at all. When I played that map for the first time, I only had the bsp file. I didn't even know about the horn sound before downloading pvk 2.3 (map and the sound included in it).
Thanks for the file. Yea, Akz's right. I don't have the sound file when I downloaded Helms Deep. I noticed the missing file during the loading of the map. It says that 'hd_horn.wav is missing' in the console.