If you want to change it, you simply take another sound file of the footstep and replace it with the other one in the audiobank\materials folder. It would have to be the same name though, such as 1stoneA1.
If you want to decrease the volume, you can download a free audio editor such as Audacity and decrease the volume from there.
I wouldn't try deleting the sound files, because they might cause an error in the game when you are trying to test or build it.
"Dad? Dad, I'm a- Ye- Not a "crazed gunman", dad, I'm an assassin! ...Well, the difference bein' one is a job and the other's mental sickness!" - The Sniper.