How would you change playback speed of all sounds at once?

--First you will need have the default playback speed of each sound, so we make a table and store them
local PlaybackSpeeds = {}

local function StorePlaybackSpeeds()
    for i, v in game:GetDescendants() do
        if not v:IsA("Sound") then continue end
        if PlaybackSpeeds[v] == nil then
            PlaybackSpeeds[v] = v.PlaybackSpeed
        end
    end
end

--Then make a function to change the playback speed of all sounds in game
local function ChangePlaybackSpeeds(NewPlaybackSpeed)
    for i, v in game:GetDescendants() do
        if not v:IsA("Sound") then continue end
        v.PlaybackSpeed = typeof(NewPlaybackSpeed) == 'number' and NewPlaybackSpeed or PlaybackSpeeds[v] 
    end
end

StorePlaybackSpeeds() -- Call the function to store the default playback speed values

-- Then are just call the functions when the Menu Opens / Close
ChangePlaybackSpeeds(0.5) -- Change the playback speed to 0.5
ChangePlaybackSpeeds('Default') -- Change the playback speed to default value

Here's the implementation on your code

local GuiService = game:GetService('GuiService')
repeat wait() until workspace.CurrentCamera
local Camera = workspace.CurrentCamera
local Blur,Color = Instance.new('BlurEffect'),Instance.new('ColorCorrectionEffect')
Blur.Size = 25 Blur.Name = 'ESCB' Color.Saturation = -.5 Color.Name = 'ESCC'

local PlaybackSpeeds = {}

local function StorePlaybackSpeeds()
    for i, v in game:GetDescendants() do
        if not v:IsA("Sound") then continue end
        if PlaybackSpeeds[v] == nil then
            PlaybackSpeeds[v] = v.PlaybackSpeed
        end
    end
end

local function ChangePlaybackSpeeds(NewPlaybackSpeed)
    for i, v in game:GetDescendants() do
        if not v:IsA("Sound") then continue end
        v.PlaybackSpeed = typeof(NewPlaybackSpeed) == 'number' and NewPlaybackSpeed or PlaybackSpeeds[v] 
    end
end

GuiService.MenuOpened:Connect(function()
    local NewBlur,NewColor = Blur:Clone(),Color:Clone()
    NewBlur.Parent,NewColor.Parent = Camera,Camera
   ChangePlaybackSpeeds(0.5)
end)

GuiService.MenuClosed:Connect(function()
    for i,v in pairs(Camera:GetChildren()) do
        if v.Name == 'ESCB' or v.Name == 'ESCC' then
            v:Destroy()
        end
    end
   ChangePlaybackSpeeds('Default')
end)

StorePlaybackSpeeds()