[Xrystal]

okay .. with the following code ( some bug fixes to spelling mistakes ) it worked. I turned nameplates always on and walked in and out of range of my other character in the guild and the sound played. It may get annoying if several guildies arrive at the same time or you are running back and forth. So you may want to consider some sort of timing system so that it only alerts you for a person once every x seconds/minutes etc.

local guildie={}
local f=CreateFrame("Frame")  >>>>> Upper case F here
f:RegisterEvent("GUILD_ROSTER_UPDATE")
f:RegisterEvent("NAME_PLATE_UNIT_ADDED")
f:SetScript("OnEvent",function(self,event,unit)
    if event=="GUILD_ROSTER_UPDATE" then
        local total=GetNumGuildMembers()
        wipe(guildie)
        for i=1,total do
            local guid=select(17,GetGuildRosterInfo(i))
            local localizedClass, englishClass, localizedRace, englishRace, sex, name, realm = GetPlayerInfoByGUID(guid) 
            if guid then
                guildie[guid]=true
            end
        end
    elseif event=="NAME_PLATE_UNIT_ADDED" then
        local guid = UnitGUID(unit)     
        local localizedClass, englishClass, localizedRace, englishRace, sex, name, realm = GetPlayerInfoByGUID(guid) 
        if guildie[guid] then
            PlaySound(416)  >>> Upper case S here
            print(name)   >>> Displays name of guild member to know who .. you can use whatever alert system you want to display the message to make it more prominent if you want.
        end
    end
end)   >>> Remember the bracket here