How do I make this script detect the smooth terrain material?

local player = script.Parent.Parent
repeat wait() until player.Character


if player.Character:FindFirstChild('Sound') then
    player.Character.Sound:Destroy()
end
for i,object in pairs(player.Character.Head:getChildren()) do
    if object:IsA("Sound") then
        object:Destroy()
    end
end


local sound = {
    current = nil
}

sound.list = {
    fsConcrete = {"rbxasset://sounds/grassstone2.ogg",1.4,.4,true},
    fsCorrodedMetal = {"rbxasset://sounds/metalgrass.ogg",1.4,.3,true},
    fsDiamondPlate = {"rbxasset://sounds/metalgrass3.ogg",1.4,.3,true},
      fsMetal = {"rbxasset://sounds/metalgrass3.ogg",1.4,.3,true},
    fsFoil = {"rbxasset://sounds/metal3.ogg",1.4,.3,true},
    fsGrass = {"http://www.roblox.com/asset/?id=16720281",1.8,.4,true},
    fsIce = {"rbxasset://sounds/icestone.ogg",1.4,.5,true},
    fsPlastic = {"http://www.roblox.com/asset/?id=121293429",1.8,.3,true},
    fsSlate = {"rbxasset://sounds/grassstone3.ogg",1.35,.4,true},
    fsGlacier = {"rbxasset://sounds/woodgrass3.ogg",1.35,.4,true},
    fsWood = {"rbxasset://sounds/woodgrass3.ogg",1.4,.5,true},
      fsWoodPlanks = {"rbxasset://sounds/woodgrass3.ogg",1.4,.5,true},
    fsSmoothPlastic = {"http://www.roblox.com/asset/?id=121293429",1.8,.3,true},

    fsWater = {"http://www.roblox.com/asset/?id=130778103",1.6,.4,true},
    fsSnow = {"http://www.roblox.com/asset/?id=19326880",1.2,.5,true},

    jump = {"http://www.roblox.com/asset/?id=130778269",1,.3,false},


    fsAir = {'',1,0,false},

    fsBrick = {"rbxasset://sounds/grassstone3.ogg",1.4,.4,true},
      fsCobblestone = {"rbxasset://sounds/grassstone3.ogg",1.4,.4,true},
    fsSand = {"http://www.roblox.com/asset/?id=131245944",1.4,.1,true},
    fsFabric = {"http://www.roblox.com/asset/?id=133705377",1.65,.5,true},
    fsGranite = {"rbxasset://sounds/grassstone3.ogg",1.35,.35,true},
    fsMarble = {"rbxasset://sounds/grassstone2.ogg",1.55,.38,true},
    fsPebble = {"http://www.roblox.com/asset/?id=133761546",1.5,.3,true},
}

math.randomseed(tick()) 

for name,data in pairs(sound.list) do
    local s = Instance.new("Sound",player.Character.Head)
    s.Name = name
    s.SoundId = data[1]
    s.Pitch = data[2]
    s.Volume = data[3]*.2 
    s.Looped = data[4]

    sound.list[name] = {s,data[2]}
end

local ground = {
    part = nil,
    position = nil
}


ground.detect = function()
    if player.Character:FindFirstChild('LowerTorso') then
        local ignore = {player.Character,workspace.CurrentCamera}
        local ray = Ray.new(player.Character.LowerTorso.Position,Vector3.new(0,-3.01,0))
        local hit,pos = workspace:FindPartOnRayWithIgnoreList(ray,ignore)
        ground.part = hit
        ground.position = pos
    end
end

local isWalking = false

player.Character.Humanoid.Running:connect(function(speed)
    if speed > 7 then
        isWalking = true
    else
        isWalking = false
    end
end)

player.Character.Humanoid.Jumping:connect(function(state)
    if not script:FindFirstChild('last_jumped') then

        local last_jumped = Instance.new('BoolValue',script)
        last_jumped.Name = 'last_jumped'
        game.Debris:AddItem(last_jumped,.5)

        ground.detect()
        if ground.part then
            if player.Character.Head:FindFirstChild('jump') then
                player.Character.Head.jump.Pitch= math.random((sound.list.jump[2]-.075)*100,(sound.list.jump[2]+.075)*100)/100
                sound.list.jump[1]:Play()
            end
        end
    end
end)

while wait(.35) do
    ground.detect()

    if isWalking and ground.part then

        local mat
        if ground.part.Name == "Water" or ground.part.Name == "Snow" or ground.part.Name == 'Air' or ground.part.Name == 'Glacier' then
            mat = ground.part.Name
        else
            mat = string.sub(tostring(ground.part.Material),15)
            mat = string.gsub(mat, "%s+", "")
        end

        if sound.current ~= sound.list["fs"..mat][1] then
            if sound.current then sound.current:Stop() end
            sound.current = sound.list["fs"..mat][1]
            sound.current:Play()
        end
        sound.current.Pitch= math.random((sound.list["fs"..mat][2]-.4)*100,(sound.list["fs"..mat][2]+.4)*100)/100
        sound.current.Pitch = sound.current.Pitch/(16/player.Character.Humanoid.WalkSpeed)
    elseif sound.current then
        sound.current:Stop()
        sound.current = nil
    end
end