Remote event not firing in a while true do loop when more than 1 player joins?

You forgot to make the Activate value false at the end.

Try using this code:

local Event = game.ReplicatedStorage.ApoxyzMusicChange

local Activate = false

game.Players.PlayerAdded:Connect(function()

    if Activate == false then

        Activate = true

        while true do

            wait(0.5)

            local randomKey = keys[math.random(#keys)]

            script.Parent.SoundId = "rbxassetid://"..randomKey

            local musicName = musicList[randomKey]
            Event:FireAllClients(musicName)

            script.Parent:Play()

            script.Parent.Ended:Wait()
            Activate = false --Added the line. 
        end
        --Put the line I added here if it doesn't work.
    end
end)

If the script above doesn't work in the way you wanted it to, try moving the line I added 2 lines below.

Hey bud, I was helping you out with your other post on this. All you need to do to fix this is add one line of code and change the order around a little. Here is your updated code:

local Event = game.ReplicatedStorage.ApoxyzMusicChange
local musicName = "" -- moved the scope outside so it can be used outside of the while loop
local Activate = false

game.Players.PlayerAdded:Connect(function(player) --Added player to capture the newly joined player

    if Activate == false then

        Activate = true

        while true do

            wait(0.5)

            local randomKey = keys[math.random(#keys)]

            script.Parent.SoundId = "rbxassetid://"..randomKey

            musicName = musicList[randomKey] -- removed local from in front of here
            Event:FireAllClients(musicName)

            script.Parent:Play()

            script.Parent.Ended:Wait()

        end
    end
    Event:FireClient(player, musicName) -- This will only send musicName to players that join. FireClient must always send the individual player, but the local script does not need to include the player, so don't change your local script.
end)

Also, I remember your other post on this, so don't change Activate back to false. You wanted the while loop to run only once as I remember. I hope this helps out! If it doesn't, please keep this post opened and add a comment under my post. I check my answers for feedback regularly.

I also noticed that you structured your code differently than what I did to prevent the Studio bug. Unless they fixed this (it's been around for a long time, so doubtful) you will definitely experience the bug as your game grows. It's ultimately your choice, but I promise it will save you some headache down the line.

[EDIT] The second problem probably should've been in another post since it was new problems based off of your changes, but I fixed it for you anyway. All I did was move some things around a little bit, you were definitely on the right track.

Here is the updated script:

local Sound = Instance.new("Sound", game.Workspace)
Sound.Name = "SongAcreol2"
local musicList = { --replace this with your module script
    ["356718047"] = "N'to Trauma",
    ["372492410"] = "Tell Me Why",
    ["2468961271"] = "Mine The Diamond"
    }
local Event = game.ReplicatedStorage.ApoxyzMusicChange
local musicName --If you have a huge song list, then it's better not to have the same information in two places. No need to set this to any value.
local Activate = false
local keys = {} 
local Change = game.ReplicatedStorage.ChangeSong
local Skip = game.ReplicatedStorage.SkipSong

for key, _ in pairs(musicList) do
    table.insert(keys, key)
end

local function Music() --Originally made this function because I thought I would need to kill multiple threads
    while true do
        wait(0.5)
        local randomKey = keys[math.random(#keys)]
        Sound.SoundId = "rbxassetid://"..randomKey
        Sound:Play()
        musicName = musicList[randomKey]
        Event:FireAllClients(musicName)
        Sound.Ended:Wait()
    end
end
game.Players.PlayerAdded:Connect(function(player)
    if Activate == false then
        Activate = true
        Music() -- Just calling a function, you can still think of the while loop as being in here
    end
    Event:FireClient(player, musicName)
end)

--[[ Commented out since I'm not really sure the difference between the skip and Change events
Change.OnServerEvent:Connect(function(player, text)
    Sound:Stop()
    wait(0.5)
    Sound.SoundId = "rbxassetid://"..text
    local musicNameInfo = game:GetService("MarketplaceService"):GetProductInfo(text).Name
    Event:FireAllClients(musicNameInfo)
    Sound:Play()
    Sound.Ended:Wait()
end)
--]]
Skip.OnServerEvent:Connect(function()
    Sound:Stop()
    wait(0.5)
    local randomKey = keys[math.random(#keys)]
    Sound.SoundId = "rbxassetid://"..randomKey
    musicName = musicList[randomKey]
    Event:FireAllClients(musicName)
    Sound:Play()
    Sound.Ended:Wait()

end)

A good rule of thumb is don't embed event-driven functions within one another. If you need several functions to use the same variables, then increase the scope of the variables to the highest scope. Be careful when doing this though, you'll need to account for the different values of the variable in the different functions.

I tested it as well in Studio for single and multiple players with the skip button, without the skip button, letting the song finish, and combinations of the three.