Did Ancient Greek cows say Μῦ μῦ?
Suda, cf. Photius s.v. Μυκηθμός
“Mukêthmos: the sound of bulls”
Μυκηθμός: ἡ τῶν βοῶν φωνή.
Cf. Schol. Q ad Hom. Od. 10.413: μυκώμεναι] βοῶσαι· μυκηθμὸς γὰρ ἡ τῶν βοῶν φωνή. Q.
We also have independent confirmation that cows may have said mu as early as the Mucynean period:
This nominal root, likely onomatopoetic from the sound of animals, has a few verbal reflexes in Greek, including μυκάομαι and μύζω. There are additional derivatives: μυκητής (“bellower”, μυκήμων “bellow”, μύκημα (“lowing, bellowing”; used of lions and thunder too). The upsilon is long to contrast with the short vowel in μύκης (“mushroom”) and Μυκήνη (Mycenae). Here’s Beekes again:
Perhaps this is not a sound exclusive to cattle, however. Consider Suda mu 1390:
“Mycalê and Mukalêsos: name for a city. It comes from the fact that the Gorgons bellowed here.”
Μυκάλη καὶ Μυκαλησός, ὄνομα πόλεως. παρὰ τὸ ἐκεῖ μυκᾶσθαι τὰς Γοργόνας.
The verb is also used to indicate the low sound of objects or the roar of a lion. See Suda, mu 1394
“Mukêsantos: “after it sounded”—Homer has “on their own, the gates of heaven sounded, the gates the seasons hold” and in the Epigrams, “after the drum sounded deeply, the boldest of the rest of the animals ran off faster than a deer.”
Μυκήσαντος: ἠχήσαντος. Ὅμηρος: αὐτόμαται δὲ πύλαι μύκον οὐρανοῦ, ἃς ἔχον Ὧραι. καὶ ἐν Ἐπιγράμμασι: οὗ βαρὺ μυκήσαντος, ὁ θαρσαλεώτερος ἄλλων τετραπόδων ἐλάφων ἔδραμεν ὀξύτερον.
And the sound moo seems to be used for non-verbal soundmaking for humans too:
Aristophanes, Thesm. 231-231
Kê: Moo, Moo
Eu: Why are you mootering? Everything has been done well.
ΚΗ. Μῦ μῦ.
ΕΥ. Τί μύζεις; Πάντα πεπόηται καλῶς.
There might be multiple layers of onomatopoetic derivatives here—one for the cow and another for the human moan, and even this is probably a simplification.
Zonaras, s.v. Μῦ (=Etymologicum Magnum s.v)
“Moo: a simple sound, this utterance imitates a moan. A moan is an echo of moo, a sound coming from the nose.”
Μῦ. τὸ στοιχεῖον, ὅτι μυγμόν τινα ἔχει ἡ τούτου ἐκφώνησις. μυγμὸς δέ ἐστιν ὁ τοῦ μῦ ἦχος, διὰ τοῦ μυκτῆρος ἐξερχόμενος.