Nope, they do not.
All these years we have been led down a farmery pathway of fabrication. I can unconditionally confirm that cows.do.not.say.'moo'.
I spent a week - well just shy of that- on a dairy farm with a field full of very vocal and loquacious cows. They had an awful lot to say....All day.
All through the night. Especially the night.
Or when the farmer drove by.....
or walked by.....
or wasn't even there...
Oh and at milking time when their herdmates are off being milked, these fishwife cows have been very voraciously putting their views forward and it went something like this...
Deep husky drawing of breath MwaaaaAAAHeeek MwaaaaAAAHeeek (a huge bellow tailing off followed with a final HA and finishing with a trebbling squeak)
HAAAAaaaaaaaaaaaaHHHH gasp for lungful HAAAAAaaaaaaaaaHHHeeeeek gasp for breath HHHHHAAAAAAAAWWWWW.
Then an equally vocal bovine neighbour responding with BAAAAAAAAHHHWWWWW wheeek(she has a strange squeaking gasp) BaaaAAAAAAAHHHWWWWWWW wkeeeek.
THEN ! in the distance and getting louder as she walked closer
bah gasp bah gasp BAH gasp BAH gasp BAAAAAAAAAHHHH
Another cow had a witty retort back MRUAWWHUH MRUAWWHUH MRUAWWHUH (she must have been breathing through her ears, she didn't stop for breath during this part of the debate).
If you look closely, cows don't press their lips together to make the MMMM sound at all, nope, they straighten their necks out, heads up, eyes wide and go HHHAAWW or BAAAHGHGH or MwaaaaAAAHeeek .
The only time I heard anything remotely like a 'moo' was when one complaining cow looked across the gate, without opening her mouth made a rumbling grumble from deep in her throat and said MMMMMMmmmmmMMMMMMMMMmmmm cough.
Oh and while we are at it - farm dogs don't go woof or say anything vaguely sounding like bark. This lot said things like....Wurrrrrrr wrrrrrrrrrrrrr or WOAH WOAH and even Hurrr Hufff HUUUFFF Huurrrr (reserved for the mobile ice cream seller) and today's dog word for the day is wurrrrrrrrrrrrr wurrrrrrrrrrr!
There. I have said it. I have broken the myth.