Footstep sounds are accomplished by creating a region using Add > PyPRP > Add a Footstep Sound Region.
List of Footstep Sounds
The following sounds are available for you to use in the "surface" property.
Footstep sounds are global resources and not all them exist in the files. Specifically, DON'T use:
This issue is especially critical for TPOTS users. Coming across one of these in any age will almost certainly crash the game.