How do you change particle size based of playbackloudness?

(I did put it in a local script)

local sound = workspace.Sound
        if sound.IsPlaying then
               script.Parent.Fire.Size = 0.5-((sound.PlaybackLoudness/1000)*1)
end

Im trying to make the size 0.5 bigger based on playbackloudness but its not working, btw I have another question here: https://scriptinghelpers.org/questions/86786/how-do-i-change-the-brickcolor-based-on-playerbackloudness

Thanks for reading.

You should use WaitForChild in LocalScripts when referencing Instances (in case they didn't load yet when you're trying to use them):

local sound = workspace:WaitForChild("Sound")

Because the script only runs on startup, the Sound probably won't be playing, that's because you have to listen for when it starts playing, with the Changed event:

local sound = workspace:WaitForChild("Sound")
local Fire = script.Parent:WaitForChild("Fire")

sound:GetPropertyChangedSignal("IsPlaying"):Connect(function()
	while wait() do
		if sound.IsPlaying then
			local PlaybackLoudness = sound.PlaybackLoudness

			Fire.Size = 0.5 - (PlaybackLoudness / 1000)
		else
			break
		end
	end
end)

By the way, the reason I'm using GetPropertyChangedSignal("IsPlaying") is because it's just a shorter replacement for

sound.Changed:Connect(function(PropertyName)
	if PropertyName == "IsPlaying" then
		---the stuff above
	end
end)

But, a simpler way of doing this is checking when the PlaybackLoudness itself changes, like this:

local sound = workspace:WaitForChild("Sound")
local Fire = script.Parent:WaitForChild("Fire")

sound:GetPropertyChangedSignal("PlaybackLoudness"):Connect(function()
	local PlaybackLoudness = sound.PlaybackLoudness

	Fire.Size = 0.5 - (PlaybackLoudness / 1000)
end)

Then, your particle's Size changes each time the PlaybackLoudness gets changed, without any inefficient loops

workspace.part.ParticleEmitter.Size = NumberSequence.new(0.5 + workspace.Sound.PlaybackLoudness / 1000)

You can change the 1000 to something else like 100 or 10 or 500 but not over 1000

or you can do this

workspace.part.Fire.Size = math.random(0.5,10)+ workspace.Sound.PlaybackLoudness / 1000 The 10 is how big is the max size of the fire and 0.5 is how small it can get