In my opinion, the metaphor in the title is one in which the girls' accents serve as a metaphor for their identity as people from the Dominican Republic.  What that means is that the book is really about the girls losing their original culture and becoming American (for better or for worse).

As the son of an immigrant, I know that an immigrant's accent is not all that separates him (or her) from American society, but it is a clear indication that this is not a "real" American.  In this book, the Garcia girls undergo a process by which they do become "real" Americans, with all the good and bad implications of that change.  They do lose their accents, but that phrase in the title is also a metaphor for all the other parts of their Dominican ancestry that they lose as well.