The method for adding sounds has changed, which I think is done in a container, but for some reason when I went looking myself for this, but couldn't find it. The alternative is to use a track object that plays a sound when a train passes over it.
There are two which I use on my routes:
<kuid:346926:14173> 14UF-TS-Kurve
<kuid:135229:100378> Invisible Horn Sequence
The 14UF-TS-Kurve is by user Alterr, and plays a tram squealing sound, which is exactly what I use it for. I think I got it as part of his U-Bahn Frankfurt Route.
The Invisible Horn Sequence is available on the DLS. I have modified this to play the horn a bit closer together by adjusting the timing.
My recommendation is to look at something like the Invisible Horn Sequence asset and create or modify something for yourself, perhaps taking one of the existing track-objects and modifying the asset for your own use.
I know this isn't quite what you are looking for, but hopefully this is helpful. nonetheless.