Area Value not changing?

There are a few small errors in your script. The main thing is that in the line you were trying to call the function, there is no function called: onPlayerRespawned(newPlayer). You can either cut it out, or create a new function within or outside of the parent function. Also, if you want the sound to play correctly, you'd have to control how many sound files are being created. You'd either create a new one every time, then delete the existing one, or create multiple sounds and make them active or deactivate at certain times. Here's what all of this should look like:

local function onPlayerEntered(newPlayer)
    local stats = Instance.new("StringValue")
    stats.Name = "Area"
    stats.Value = "Spawn"
    stats.Parent = newPlayer    
end

function onPlayerRespawned(newPlayer)
local music = newPlayer:FindFirstChild("Sound")
if music == nil then
    local music = Instance.new("Sound", newPlayer.PlayerGui) 
    music.Looped = true
    local Id = "rbxassetid://" 
    music.SoundId = Id.."319731744"
    music.Volume = .5
    music:Play()
elseif music ~= nil then
    music.Looped = true
    local Id = "rbxassetid://" 
    music.SoundId = Id.."319731744"
    music.Volume = .5
    music:Play()
end
local stat = newPlayer:FindFirstChild("Area")
if stat.Value == "Lobby" then
    music.SoundId = Id.."326012211"
elseif stat.Value == "Hi" then
    music.SoundId = Id.."314069452"
end
end
function onPlayerEntered(newPlayer)
newPlayer.Changed:connect(function (property)
if (property == "Character") then
    onPlayerRespawned(newPlayer)
end
end)
end

game.Players.ChildAdded:connect(onPlayerEntered)

Hopefully this gave you a start to your script. I've only edited the parts that need working on, so not everything everything is garenteed to work. If have any questions, feel free to put a comment below. Otherwise, I have some links that'll hopefully better explain to you the kind of concepts I'm trying to cover. YOUR WELCOME!!

LINKS:

Roblox Wiki: http://wiki.roblox.com/index.php/Scripting

Sound: http://wiki.roblox.com/index.php?title=API:Class/Sound

functions: http://wiki.roblox.com/index.php?title=Function

The Player : http://wiki.roblox.com/index.php?title=API:Class/Player