If it's not in the SOU pack, try downloading the SP F's on JR too. They also use that hornsound. If you can't find it in there either, go here http://www.trainzkuidindex.com/eng/Assets/search, and enter the full kuid. It should give you a list of sites and subsequent links to content that should also contain this asset. You may need to register for the above site, it is free to do so.
If all else fails and the horn isn't in any of these, you can try changing the hornsound to one more to your liking via the config file.
If you find missing dependencies, this is a great link. Registration is required, but I find it well worth it. Not EVERYthing is on this, but a large chunk of it is. Once you type in the KUID, it will send you to a direct link that contains that dependency. Best of luck finding it. Just noticed it was posted in the comment above lol.