Subject: Why do identical twins have different sounding voices?

I understand that environment and heredity both play roles in the make up of a 
person and that while genetically identical, twins are different. But, from my 
understanding, the make up of one's voice comes from the length of one's vocal 
cords and is not affected by environmenal influences. If that's true, the 
vocal cords should be determined by genetics alone, and if twins have the 
exact same genes, shouldn't their voices be identical as well?

Re: Why do identical twins have different sounding voices?