Exclude own messages from triggering add-on sound effect

Question

So I've been working on an add-on that will allow me to have a sound play when a guild message comes in. I have been successful in getting the sound to play when messages come into guild chat with one problem: it also plays when I put a message in guild chat.

I need some help trying to figure out how to omit myself from it playing a sound when I put messages in.

This is for vanilla WoW 1.12.1 (not retail or classic).

Here is my code:

local CHAT_MSG_GUILD = { "" };

local CHAT_MSG_GUILDAlertFrame = CreateFrame("Frame");
local playerName = UnitName("player");

CHAT_MSG_GUILDAlertFrame:SetScript("OnEvent", function()
    if arg1 ~= nil then
        for i=1,table.getn(CHAT_MSG_GUILD) do
            if string.find(arg1, CHAT_MSG_GUILD[i]) then
                PlaySoundFile("Interface\\AddOns\\HelloWorld\\Sounds\\wisp.wav")

                return;

            end
        end
    end
end);

CHAT_MSG_GUILDAlertFrame:RegisterEvent("CHAT_MSG_GUILD");