How would you pronounce N.B. (nota bene) when reading aloud?
You pronounce N.B. (also written as NB or n.b., nb) as
/ˌenˈbiː/.
(MacMillan). See also Cambridge. In other words you say the names of the two letters, just like for e.g. (also eg) ( /ˌiː ˈdʒiː/). (MacMillan) You can also say Note or Note well, which parallels saying for example for e.g.
NB that the NB is from the Latin nota bene although the phrase is the same in Italian.
See also How are "i.e." and "e.g." pronounced?