You know, I'm beginning to think that this may be a Northern accent thing. I speak with a Southern dialect, and it seems to be a reoccurring issue in the comments. Ever listen to Vietnamese songs or even your parents sing karaoke (totally my mom)? You know how Canadians, Americans, Britains, and Australians speak with different accents but they sorta sing with the same accent? I really think "một" is pronounced like that because that's how the Northerners say it. Just a guess though! Figured I could give my opinion/observation haha!
I think that's a good theory, however I live in Hanoi and they don't say 'một' like "mộn" - I think it's a problem with the audio for this sentence
I read in a different comment section, that the speaker most likely comes from the Northern side of Vietnam (Even more in the north than Ha Noi and that this is the reason for his accent.)
I think "cái" sounds just fine. Could be the quality of the audio or that we all have different sensitivities to how something sounds?
I didn't know "cây" is the classifier for scissors. My Vietnamese is so basic (Viet family, born and raised in Canada), so I use "cái" for so many things. I read somewhere that it's considered a generic classifier and that's why "cái" works in many cases.
In English, some singular things are considered a pair. "A pair of glasses" is a single object. "Ten pairs of pants" refers to ten objects.