No Sound Playing When Part Touched? (part 2)

First of all, you're trying to play the sound in the PlayerGui. You can't do that. PlayerGui is just like a storage for the rest of ScreenGui's and Frames. You would need to parent the sound to a Frame that is located inside a ScreenGui in the PlayerGui. Let me fix up the code for you, because there are just simple mistakes.

local LobbyMusicPart = game.Workspace.LobbyMusicPart  
local playergui = game.Players.LocalPlayer:WaitForChild(“PlayerGui”)  
local LobbyMusic = Instance.new(“Sound”)
LobbyMusic.Name = "LobbyMusic"
LobbyMusic.Parent = playergui:WaitForChild("ScreenGui").Frame -- You need to obviously create these, feel free to change the names, obvs.
LobbyMusic.SoundId = "http://www.roblox.com/asset/?id=2458556355 -- Used another method for getting an asset.

LobbyMusicPart.Touched:Connect(function(hit) -- This is a function that runs if a part is touched.
if hit.Parent:FindFirstChild("Humanoid") then -- Checks if the touched part is a player.
LobbyMusic:Play()
end
end)

LobbyMusicPart.TouchEnded:Connect(function(hit) -- This is a function that runs when a part stops touching the music part.
if hit.Parent:FindFirstChild("Humanoid") then
LobbyMusic.Playing = false
end
end)

Hope this helps.