First thing i notice: UnitDebuff is not an event. Events are written in all capital letters. The event you are looking for is UNIT_AURA.
this should do it:
Lua Code:
local Blinding_Peck = GetSpellInfo(195561)
local frame = CreateFrame("FRAME")
frame:RegisterUnitEvent("UNIT_AURA", "player", "party1", "party2", "party3", "party4")
frame:SetScript("OnEvent", function(self, event, unitID)
if UnitDebuff(unitID, Blinding_Peck) then
PlaySoundFile("Interface\\AddOns\\Segullspeck\\media\\Seagullspeck.ogg", "Master")
end
end)