[SDPhantom]

Here's what I've come up with along with some tweaks and optimizations.

local AddOnPath=("Interface\\AddOns\\%s\\"):format(...);--  Dynamically fetch our addon path
 
local Frame=CreateFrame("ScrollingMessageFrame",nil,UIParent)
Frame:SetSize(450,200)
Frame:SetPoint("CENTER")
Frame:SetFont(AddOnPath.."Res\\PHA_.TTF",30,"THICKOUTLINE")
Frame:SetShadowColor(0.00,0.00,0.00,0.75)
Frame:SetShadowOffset(3.00,-3.00)
Frame:SetJustifyH("CENTER")
Frame:SetMaxLines(2)
--Frame:SetInsertMode("BOTTOM")
Frame:SetTimeVisible(2)
Frame:SetFadeDuration(1)
 
Frame:RegisterEvent("PLAYER_LOGIN")
Frame:RegisterUnitEvent("UNIT_PET","player")
Frame:RegisterUnitEvent("UNIT_HEALTH","pet")
 
local LowHealthThreshold=0.35;
local LastHealthValue=0;
Frame:SetScript("OnEvent",function(self)
    if UnitExists("pet") then-- Check if pet exists
        local cur,max=UnitHealth("pet"),UnitHealthMax("pet");
        if cur and max and max>0 then-- Check if we have health data
            if cur<LastHealthValue then--   Has health decreased?
                if cur<=0 then--    Is pet dead?
                    PlaySoundFile(AddOnPath.."Res\\PHA.ogg")
                    self:AddMessage("- PET DEAD -", 1, 0, 0, nil, 3)
                elseif cur/max<=LowHealthThreshold then--   Is pet low health?
                    PlaySoundFile(AddOnPath.."Res\\PHA.ogg")
                    self:AddMessage("- CRITICAL PET HEALTH -", 1, 0.5, 0, nil, 3)
                end
            end
 
            LastHealthValue=cur;--  Update tracking value
        else LastHealthValue=0; end--   Reset to zero if health not available yet
    else LastHealthValue=0; end--   Reset to zero if no pet
end)