Just another thought here......

What is she like personality wise?? My DS is younger but also does not say much, he tends more so to communicate with his eyes and body language. He has always been a deep thinker, very sensitive and i feel in tune with things around him. In doing research i came accross some angel therapy and the generation of the Crystal Children. One characteristic is that they are delayed or late speakers and often have large mesmerising eyes (that was the first thing that clicked for me because i can't leave the house without strangers commenting on his eyes).

Anyway some info here if you are interested:

Crystal Children Article

I certainly would still pursue looking in to speech therapy etc, but i just thought i'd post this as it may be relevant and i found it quite interesting