A quote from my answer to the question "Why is 눈 pronounced the way it is?"
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.
Also, there is some regional variation: 경기도 (Seoul and around Seoul) has a much greater extent of denasalisation than 경상도 (Busan and Daegu etc.) where nasality is retained. However, there is also a considerable generational difference; older Busan speakers seem to never denasalise, whereas younger Busan speakers 40-70%.