I was having the exact same problem. The only solution I could think of is to place an invisible mesh on top of the puddle so it will play the appropriate sounds. I just modeled a simple “collision mesh” for each of my decals and then placed them on top of the puddle decals in my level, made their material use the appropriate physical material and set them to be hidden in game. It’s not perfect, but since there doesn’t seem to be collision for decals, it’s the easiest solution I could come up with.
Just be sure to set the collision of the mesh to “Use Complex As Simple” so that it’s appropriately accurate. Since the mesh is so low poly (only 154 polys) it’s not going to adversely affect performance at all.