You need to build up a table with all options in advance. Example:
local soundtable = {
  stone = {
     "vispath:sounds/steps/stone1.ogg",
     "vispath:sounds/steps/stone2.ogg"
,
     "vispath:sounds/steps/stone3.ogg"
,
     "vispath:sounds/steps/stone4.ogg"
  },
  wood {
     "vispath:sounds/steps/wood1.ogg",
     "vispath:sounds/steps/wood2.ogg"
,
     "vispath:sounds/steps/wood3.ogg"
,
     "vispath:sounds/steps/wood4.ogg"
  }
}
local v = getObject("Values[FootstepSound]"):getStr(VValueString)
local r = math.random(1,5)
startSound(soundtable[v][r], {flags=1, volume=20})