The late Professor Charles Richet during a seance tried to answer a similar question through Flint's direct voice in the book 'Voices in the dark'. (P.291). quote: " He (Richet) grew quite testy when someone suggested that sometimes the voices of communicators from the spirit world did not sound exactly the same as their voices during their life and he said it was hardly likely that they would sound the same, seeing they were not using the same vocal organs they had in life. He added that we must take in to account , also, that the communicating spirit was trying to concentrate on three different things at the same time while communicating." He explains that the three things are lowering his own frequency to the lower one of the earth, but at the same time remember what his his voice sounded like in his earthly life, and also recapture memories of happenings which will give proof of his identity to the person with whom he wishes to communicate, while speaking in to the voicebox formed from ectoplasm. Richet said: " The miracle is that we can communicate at all." That doesn't quite answer your question, zzzzwhat, but it is obviously a complicated process and the mechanisms are not properly understood. There are examples in the book where the voices were said to be just like in earth life, and evidence of identity was given.