[SDPhantom]

Here's an example I came up with. It's based off how ReputationFrame updates and keeps track of rep by ID to avoid name collisions.

local TrackedFactions={};
local EventFrame=CreateFrame("Frame");
EventFrame:RegisterEvent("PLAYER_LOGIN");-- Player login
EventFrame:RegisterEvent("UPDATE_FACTION");--   Kill rep
EventFrame:RegisterEvent("QUEST_LOG_UPDATE");-- Quest rep
 
EventFrame:SetScript("OnEvent",function()
    for i=1,GetNumFactions() do
        local _,_,newstanding,_,_,_,_,_,_,_,hasrep,_,_,faction=GetFactionInfo(i);
        if hasrep and faction and (newstanding or 0)>0 then--   Make sure we have the info we need and standing isn't UNKNOWN (0)
            local oldstanding=TrackedFactions[faction];
            if oldstanding and oldstanding<newstanding then--   Check if standing went up (allow same code to initialize tracking)
                PlaySoundFile("Interface\\AddOns\\YourAddOn\\YourSoundFile.ogg");
            end
 
            TrackedFactions[faction]=newstanding;-- Update standing
        end
    end
end);