[1] That may be due to speech pathology but also occurs when the sinuses are blocked from a common cold, when it is called a nasal voice, which is not a linguistic term.
[4] When one speaks with a cold, the nasal passages still function as a resonant cavity so a denasalized nasal [m͊] does not sound like a voiced oral stop [b], and a denasalized vowel [a͊] does not sound like an oral vowel [a].
Similarly, several languages around Puget Sound underwent a process of denasalization about 100 years ago.
Except in special speech registers, such as baby talk, the nasals [m, n] became the voiced stops [b, d].
Something similar has occurred with word-initial nasals in Korean; in some contexts, /m/, /n/ are denasalized to [b, d].