[ChilenoAlemanCanada]

Originally Posted by Rusty

In many languages (although not in English, in most instances), the letter n is pronounced as if it were the letter m when it is followed by a labial consonant (i.e., b, m, p, and v). There is also a spelling convention, which is what you were stating, that the letter n is changed to an m before those consonants.

The spelling convention isn't always followed, like in the word inconveniente, but the pronunciation rule is. This word is pronounced as if an m appeared before the v.
Here are more examples:

Convencer is pronounced combencer.
Conmigo is pronounced commigo.
Enmascarar is pronounced emmascarar.
Sinvergüenza is pronounced simbergüenza.
Tan bien is pronounced exactly like the word también.
En piezas is pronounced exactly like the word empiezas.

I dunno, I don't think that would apply to Latin American Spanish. Are you from Spain, by any chance?