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I guess one would beep beep like a sheep, by having said sheeps internal organs removed and replaced by electronic equipment that will make said sheep stronger and faster, and then wipe their brains empty, and install the doctrine of an evil millitary alien occupying force into the brain so the sheep becomes a tool of death and destruction for the alien overlords, who use the sheep to brutally murder any resistance forces that the human species has put up, whilst the sheep will make a "beep beep" noise, as it stands watch over the ashes of human civilisation.
But that's just the imagine that pops up into my mind when I think of a "beeping sheep"... Possibly because half-life 2 had quite an impact on me...