-- This could be anything in your sounds-folder.
local song = "joinsounds/joinsound.mp3";

if(SERVER) then
    -- Send this script to the players.
    AddCSLuaFile("joinsong.lua");

    -- Send the song to the players.
    resource.AddFile("sounds/" .. song);

    -- When a player spawns for the first time..
    hook.Add("PlayerInitialSpawn", "Play some song", function(ply)
        -- Send them an usermessage telling to play the song.
        SendUserMessage("playjoinsong", ply);
    end);
else
    -- When we recieve the usermessage to play the song..
    usermessage.Hook("playjoinsong", function()
        -- Play it.
        surface.PlaySong(song);
    end
end