^I get hornsounds all the time from videos, Youtube or otherwise. I use a program called Audacity ( http://audacity.sourceforge.net/ ) which can be used to record and modify any sound from the internet.
As for copyright infringement, I can't believe anyone would go to the trouble of taking anyone to court for stealing a horn sound! They'd have to absolutely prove the accused actually recorded their "stolen" sound. There are thousands of trains out there every day blowing their horns! Besides, maybe I missed something, but I can't see any copyright notices anywhere on Youtube:
"lets say I'm making a locomotive, I also have my own custom hornsounds for it, I also capture some video's and put it on youtube, someone records the horn from my video, uploads it to DLS, after I have my model made and I have it uploaded on DLS people are going to think I stole the hornsound as it's the same from the one someone uploaded before me"
Thats my thoughts on this, if you do copy a hornsound from youtube should message the person and ask "can I use this for trainz sim and upload the sound to DLS?"
Or you can do what I did, leave a tape recorder running out in the yard somewhere "for me it was Newport Workshops" well I actually had it on the deck of the Y Class or T class ;P