I am acquainted with one person who once male now passes as female but I don't know whether she has been surgically re-assigned. I only mention that in that surgery and large doses of hormones might I imagine change voice timbre.

She plays a lovely guitar (fitted with hipshots for speedy drops into Drop D, Double Drop D and DADGAD) quite well and has a wonderful speaking voice: so wonderful that before I knew the facts (or at least those of them that I do know) I asked her why, with such a lovely speaking voice, she did not sing. Well, I have now heard her sing, and her voice is not conventionally female, but not delivered as conventionally male either, and is very musical. Given the laws of supply and demand I don't know if she would "make it commercially" as a contemporary/traditionalstyle singer songwriter but she is plenty good enough for support slots and as likeable to listen to as many semiprofessional folkish singer/guitarists.