Re: Why do identical twins have different sounding voices?

Hi Jessica!
The answer to your question is that, in addition to genetics, there are
also other factors involved in creating our adult voice.
These include hormones (particularly at puberty); the influence of external
factors (particularly smoking); and the range of sounds we hear and produce
(particularly shouting or singing).  Now, the genetics in identical twins
are the same, but the hormones (even in the womb) are NOT!
That explains why identical twins may be both right and left handed; have
different fingerprints and may well have different skills and seek different
careers.
A much better and more detailed answer to this question is already on
Madsci here: http://www.madsci.org/posts/archives/2006-02/1139766670.Ge.r.html

I hope that helps - the real answer to many of these questions is that
there are complex interactions between our genes and our environment which
we are only just beginning to unravel.  Once we do, we may be some way to
explaining why some smokers live to be 90, whilst most die much younger
from lung or heart diseases and why some people have a genius for music,
maths or art when their family history has no suggestion that these factors
have appeared in their ancestry.

Hope that helps a little!
HD2