What? Are they having a laugh?
OK, so I was always taught that the "barb" in "barbarian" was the same as the one in "barber", meaning "beard", but I was aware that this is probably wrong. No way does it come from the noise that sheep make, though!
Firstly, the ancient Greeks kept many more goats than sheep. Greece does not have sheep-friendly terrain. Why would they choose sheep noises to represent the speech of foreigners?
Secondly, the word goes back to beyond Greek times to more general Indo-European roots. It did mean "people with unintelligible speach", but the Sanskrit interpretation of the notion means "stammering" — nothing to do with the noise sheep make.
Damned lying calendars! If I can't trust the definitions, how do I know I can trust the dates?