I may not have explained it well, and don't know much about sounds in the game, so maybe I'm not understanding it.
I am using the individual mods downloaded from Nexus.
The box truck horn works. In vehicles.xml there is the property <property name="hornSound" value="BT_horn"/> and a corresponding sound data node in sounds.xml <SoundDataNode name="BT_horn">.
For the dirt bike there is no sound when pressing the horn button. Vehicles.xml has <property name="hornSound" value="horn_loud"/> but there is no corresponding "horn_loud" sound data node, not in the mod or vanilla. There is <SoundDataNode name="DBhorn_loud">, and if I change the hornSound property to use this node then the horn works.
I normally don't use the horn for anything but lately I've been using the horns open doors mod. It still works with no sound, but it's nice to have the audio cue that I hit the right button.
The icons I remade do show up better, but they don't look good, more like a grainy yellow silhouette. I do find navicons useful on the compass when trying to find my way back to my vehicle. I may see if I could do something better but graphics isn't a strong point for me either.