Working ladders (Source 2)
Adding Sound to Ladders
In 
Source 2, adding sounds associated with a player climbing or making contact with a ladder can be accomplished using trigger_multiple.
First, create a point_soundevent, and place it centered on the ladder object. Use the "Sound Event Name" parameter to select a sound to be played, such as a ladder step. Using "Name" parameter, write a unique name for the entity.
Create a trigger_multiple that closely encompasses the ladder object. Open the object's properties, and under "spawnflags", change "Delay Before Rest" to 1. In the "Outputs" tab, create three outputs:
The "Target Entity" name should be the name of the point_soundevent you created.
This process should result in a ladder that creates noise upon a player moving up or down across the trigger_multiple, and stops making noise if the player stops moving. Adjust the delay timings as necessary to accompany player movements and animations.
This process should be repeated uniquely for each instance of a ladder on a map, to avoid sounds being played from an incorrect location.