SoundId needs a string and not an object?

local relaxid = "231311947"
function playmusic(id, replay)
    playing = true
    if game.Workspace:FindFirstChild("music") then
        game.Workspace:FindFirstChild("music"):Stop()
        game.Workspace:FindFirstChild("music"):Destroy()
    end
    local music = Instance.new("Sound", game.Workspace)
    music.Name = "music"
    music.SoundId = id
    if replay == true then
        music.Looped = true
    else
        music.Looped = false
    end
    music:Play()
end

playmusic(relaxid, true)

The problem with this script is that it says that music.SoundId needs to be a string, and that id is an object. How would i turn id into a string?