"Footsteps" are the result of "havok material" collisions between the material underfoot and the "footwear". However, it is dependent upon the "action" and not what is worn by the Actor. Please see the "Collision" section of the wiki "Getting started creating mods using GECK" article as a starting point.
However, before you dive into modifying meshes, are you sure the problem is not due to a "mod conflict"? Are you using anything to resolve "record level conflicts" (such as a "merge patch") and have you tried isolating potentially conflicting "power armor" mods from each other to determine if that might not be the source of your problem? I ask because this is not a universal complaint. I haven't seen it appear previously in the last 5 years.
If you aren't aware of a "merge patch" file, please see the 'Third Rule: The Rule of One' and 'Merge Patch File' sections of the wiki "FNV General Mod Use Advice" article.