The added wooden ladder variants do not have climbing sounds like the default ladder does. I assume the fact that the iron ladder has no sounds is intentional, as the sounds present in the game are wooden.

Tacked on suggestions: Perhaps iron ladders could use iron footstep sounds. Ropes could use wool footstep sounds.

I've tried to figure out what causes this and frankly I have no idea.

Enhanced Mushroom's added ladders don't have this issue. Maybe the devs for that could help? I don't know how much devs help each other so if this is a weird concept I apologize.

I noticed iron ladders make stone sounds when placed and broken as opposed to the sounds shared by other iron blocks. Additionally, they make wooden sounds when placed in the way that builds a ladder downward from the block being clicked.