I have the intriguing question about PlaybackLoudness because I'm making a speaker and the circle in the middle's size is supposed to change synchronously with the Loudness of the sound
Of course it led to some horrible results. This is the last place i could think of getting help from and it's 2AM why am i awake
local Sound = workspace:WaitForChild("Sounds").Sound
local Loudness = Sound.PlaybackLoudness / 10
Sound:Play()
wait(2)
while wait() do
script.Parent.parta.Size = Vector3.new(Loudness, Loudness, Loudness)
end

2 comments
I don't know what exactly to change with your scripting, but I do know that PlaybackSpeed is used to increase the speed and pitch of a song, so unless you are trying to do that, it sounds like you want to do something like Sound.Volume = 1.5
No, he is correct about playback loudness