"Because it was an allophone of /n/ before final [Ng] was simplified
to [N], presumably related to the reduction of final [mb] to [m] (as
in 'lamb') and vacillation in the reduction of final [nd], which
allowed 'parasitic' [d] as in English _sound_ from Old French from
Latin _sonus_." Richard WORDINGHAM, ENGLAND
 
The parallel between /mb/ > /m/ and /Ng/ > /N/ is very good. Congratulations.
/nd/ > /n/ occurs at least in one word : "handsome".
 
Jean-Paul G. POTET, FRANCE