The word 눈 is pronounced differently to what I expect it would, with regards to the first consonant. Fluent Forever mentions that pronunciation rules for Korean are fairly complex, but I don't understand this particular case, and I couldn't find anything by doing a search. What rule explains its pronunciation?
Link to krdict entry, where the first consonant sounds more like a "d" sound rather than an "n". Some entries on forvo also have this pronunciation.
The phenomenon you're hearing has been described not just in academic literature but also in more modern learners' guides to the Korean language. It is an example of initial denasalisation. Basically, the nasals ㅁ and ㄴ, phonemically /m/ and /n/ respectively, tend to denasalise to a heavily voiced [b] and [d] in initial position, and especially before ㅜ /u/.
This was reported as far back as 1924 by Daniel Jones himself no less, with this phenomenon happening before /u/. It seems to have generalised now. The 2011 study showed that they are perceived as nasal consonants by Korean speakers but as non-nasal /b/ and /d/ by English speakers.
