I think this 'n' sound is just something strange and guttural. The tone pitch in một is right for me, but the duration it is near the hỏi tone (longer than nặng), and it is followed by this strange guttural sound (perhaps not with the mouth closed but the tongue pressing above to generate the glottal stop with the airflow stopped producing this strange final sound) that seems to be a variation but I am not really sure of that.
To adapt our ears to a very different phonology, I recommend to practice listening some videos (I do not know if you already are involved with Vietnamese dialects or you know Vietnamese people as some users in the course. I also see that some Vietnamese native speakers are trying to help in the course):
Easy Vietnamese 1 - What's typical Vietnamese?
(I also recommend to use the speed configuration to listen to the slow speed (0.75 or 0.5) before the normal speed. And read only the Vietnamese subtitles before reading the English. One more thing, some Vietnamese words do not appear in the subtitles during all the speech, perhaps to show more readable sentences.)
Many speakers used to add a pause between sentences (something like umm or uhhh... in English, and the reporter says yaaaa... or yeah!) and I like more the pronunciations of speakers after 2:44 but all of them are really interesting and very fast, especially the reporter's questions. ;)
The word một appears in the subtitles as 1. For example the speaker in 1:18 seems to pronounce m and t touching the lips a little, but I think when t should be pronounced the mouth is still open or perhaps the t is pronounced as b: