This is what Lee had to say about it in October last year.
Quote: "LeeBamber:
As far as I know, and I should know, there was never any code which detects the material type of terrain painted surfaces, and so ALL footfalls on terrain would default to the grass sound effect. The Building Editor, on the other hand, has full control of material indexes for textures and should work fine. Given the close relationship in the way the two texture atlases are presented, it does make sense to add this functionality for terrain textures, but it's not part of the current workload. I think it can probably make it into a sooner update without resorting to the voting board as I think most of the code is straightforward and just needs to make sure it does not impact performance in any noticeable way (i.e. there is a potential read of the terrain texture each time the player (or character) stomps down on the floor, but I have a feeling I will move the data to a very fast an optimized memblock, and move the functionality to a LUA command so it keeps it well away from more internal hard coding
"
https://forum.game-guru.com/thread/218736#msg2587279
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