Why does he pronounce the "tại" like "tãi?" Is there a tone change when the sentence starts with a 6th tone? I notice he also does this when the sentence starts with "mọt," pronouncing it more like "mõt."
In the Northern dialect, the ạ gets a creaky voice, which is similar to the Northern pronunciation of ã . I do think the audio is a little messy here, but it does stick fairly closely to the pronunciation. The một issue is just an error
I think it is a mistake in the audio. The very next question I had on DL was Tại vì tôi and it pronounced it clearly i.e "Tie vee toy" without what you refer to as the 6th tone - dấu ngã. I can only assume you are referring to đấu ngã as the sixth tone as there is no official order of the tones at least not that I am aware of.
This is new to me. I've always heard it as tại sao? and then the answer would be 'vì .....'
You can say Tại sao? or Vì sao? or Tại vì sao?, theyre all the same. And the answer can be Tại.... or Vì....
"Tại vì sao?" is identical to "vì sao?" However, Tại = Caused by.
So there is a sense of "who/what's causing it?"