I'd just like to add on that the reason the dot makes that sound is that [b] is a labial consonant, pronounced with the lips, and it's far easier to pronounce [m] instead of [n] before consonants like [b] or [p]. There are many sounds the dot can make.
Generally, it makes the [म/m] sound before labial consonants such as [ब/b], [प/p], [भ/bʱ], and [फ/pʱ]
The [ङ/ŋ] sound (like in 'ink'), before velar consonants such as [क/k], [ग/g], [ख/kʱ], [घ/gʱ]
The [ण/ɳ] sound before retroflexes such as [ट/ʈ], [ड/ɖ], [ठ/ʈʱ], and [ढ/ɖʱ]
The [ञ/ɲ] sound before palatal consonants such as [ज/dʒ], [च/tʃ], [झ/dʒʱ], and [छ/tʃʱ]
The [न/n] sound before denti-alveolar consonants such as [त/t], [द/d], [थ/tʱ], [ध/dʱ]
And it's generally not used before [य/j], [र/r], [ल/l], [व/ʋ], [श/ʃ], [ष/ʂ], [स/s], and [ह/ɦ]
There are some other consonants I didn't mention, but I covered all the types of consonants.
You shouldn't work too hard to pronounce all these, though. Most of them come naturally when spoken and are found in English too(co[nch], ba[nk], ra[nt], a[mp]ersand.)