if I were to make a music command how do I make it work?

lplayer.Chatted:Connect(function(msg)
if string.sub(msg, 1, 7) == (prefix.."music ") then

end
end)

so I get up to this point then I don't know what to do where it gets the ID from you saying :music [ID} then it plays. Anybody know?

Since you want to do this from a local script, you won't be able to create a new sound instance for everyone, so no one else will be able to hear the music. So, what you need to do is use remote events:

your local script:

local RE = game.ReplicatedStorage:WaitForChild("MusicEvent")
local localPlayer = game.Players.LocalPlayer

localPlayer.Chatted:connect(function(Message)
	if string.sub(Message, 1,7) == ":music " then
		local ID = string.gsub(Message, ":music ", "")
		RE:FireServer(ID)
	end
end)

Server script inside of server script service:

local RE = Instance.new("RemoteEvent")
RE.Parent = game.ReplicatedStorage
RE.Name = "MusicEvent"

RE.OnServerEvent:Connect(function(player, musicID)
	if workspace:FindFirstChild("Sound") then
		local Sound = workspace.Sound
		Sound:Stop()
		Sound.SoundId = "rbxassetid://"..musicID
		Sound.Looped = true
		Sound:Play()
	else
		local Sound = Instance.new("Sound")
		Sound.SoundId = "rbxassetid://"..musicID
		Sound.Looped = true
		Sound.Parent = workspace
		Sound:Play()
	end	
end)

If this answers your question then mark it as the accepted answer. I can also clear up any further questions you might have about this.

I made it so it's possible for you to add other commands, if "music" is not where you're going to stop at

local Players = game:GetService("Players");
local admins = {"tantec"};
local sound = Instance.new("Sound");
sound.Parent = workspace;
local prefix = ":";
local commands = {
	music = function(id)
		sound.SoundId = tonumber(id);
		sound:Play();
	end;
};

for i,v in pairs(Players:GetPlayers()) do
	local check = false;
	for a,b in pairs(admins) do
		if v.Name == b then
			check = true;
		end;
	end;
	if check == false then continue end;
	v.Chatted:Connect(function(msg)
		for c = 1,#msg do
			if msg:sub(c,c) == prefix then
				local tab = string.split(msg:sub(c+1, #msg));
				local cmd = tab[1]:lower();
				local arg = tab[2];
				pcall(function()
					commands[cmd](arg);
				end);
			end;
		end;
	end);
end;