I'm still unable to replicate the problem. I thought at first it might be some kind of variable that was being set and then not being changed after climbing the ladder, but that's not how the engine determines footfall sounds as far as I can gather.
I'd try putting a solid platform beneath the ladder temporarily to see if the problem persists in cases where you go from solid ground to the ladder. If so, I'd move/delete the water below and test it to make sure the sounds upstairs are corrected.
Is the ladder touching the water? Ladders are weird, and that might be affecting it (somehow
) - I know ladders shouldn't touch anything at all to have them function properly, but to be honest I've never tried partially submerging one.