I'll share an MTF persective on this...
Of course, the MTF voice does not change on HRT and has to be changed by the user. When I changed my speaking voice, I simply raised my voice into falsetto, lowered it to a reasonable tone and started from there. To me, it was no more difficult than this to get started. Of course at first it was breathy and would sometimes 'drop' and speaking louder was a challenge - but this simply came with increased 'exercise' of using my voice in this register. I am not attempting to show how 'good' I am - I believe that anyone can do this - and even though my young age does help, I have seen older people able to master it well. To me it is far simpler than having session after session of voice therapy, although I imagine it can be an aid to use alongside self-practice.
Perhaps the reason for my perspective is my singing background. Pre-transition, I was classified by my singing teacher as a baritone (between tenor and bass singing voices, so certainly not a high male voice). I am now classified as a contralto/mezzo soprano (meaning I can sing in the lowest female range up to the medium high range, although not the highest soprano range). This came initally from being able to strengthen my upper "falsetto" range, which like most male voiced people at first, sounded breathy and weak (try singing a song from an average female singer, if you are mtf, and you will know what I mean). However, what I found to be a great benefit were the lessons learned from singing - that is, to develop head reasonance to strengthen the upper range. Now I do NOT sing in falsetto, that breathy soft sound at all. What I sing in for the high range is 'head voice' which has vocal chord closure and is not breathy at all. This sounds like vague 'singing-speak' so to put it in clearer terms, it means to focus the voice 'up' into the head (difficult to explain without showing it) but the best way to explain it is to pretend to impersonate an opera singer and the tone they use when singing. When you try this, with practice, you will 'feel' how the voice can resonate more clearly in a higher tone. It also comes from 'support', that is to support your voice with your diaphragm, and breath control, which means a controlled releasing of breath when you speak/sing. The way to actually do this I compare to almost holding your breath - in particular when you sing a long note (or you can try sounding a long note in speech) sound it whilst almost trying to hold as much breath in as possible - like holding your breath. Of course some breath will come out as you are using your voice, but if you try as much as you can to 'hold your breath', you will find that you have a much less breathy quality to your voice, and in the case of singing, be able to hold notes much longer and maintain more control over tone, vibratto, quality etc.
I found that eventually I astonished myself with how loudly I could sing upper tones - which I had previously thought would be impossible. There are some limitations - my upper range will never be as strong sounding as my lower range (modal voice) although I have been told my lower range is particularly strong. However, I have made the most of this and realised that instead of trying to speak/sing like any other female, I would make the most of the uniquness of my voice to produce a sound which is androgynous - using a thick strong lower register with a purer upper register of voice. I have also found mixed voice a great way to sing much higher pitches strongly and with little effort - it comes by using the gentler head voice and almost bringing a bit of the chest voice back into it (almost 'shouting into' the head voice with control).
I highly recommend singing for people interested in voice - I feel I have come a long way in 5 years in terms of voice and still have much room for further improvement (particularly in singing) looking forward. I hope that you will find that speaking and singing in a higher and/or more feminine register is entirely possible (I thought it would be impossible for me to realistically sing like a mezzo soprano without sounding like the bee gees).
Below are three songs I have sung using a variation of registers - I have made the most of the fact that my voice is different from other females, and hence I do not concentrate on making my voice sounding perfectly female, but instead sounding androgynous, as voices where you can't quite tell the sex of the singer have always fascinated me. It seems, however, that my voice 'passes' when singing, although it has masculine elements to me, which is fine, as some famous female singers have masculine sounding voices.
A good song to start with which is quite easy to sing is 'Je ne regrette Rien' as I sing here:
Claudia - Je ne regrette Rien at South Bank (I had a cough here hence the voice almost cracking near the end!)
This song makes the most of my trans voice - the higher register, but also some very low notes that would be impossible for 99% of genetic females (especially ones my age)
Claudia - SOS Sous un terrien en détresseAnd finally, a song in the upper register, mainly using head voice - such a way of singing can be achieved by gradual increased use of the upper register so the voice becomes 'used' to it and singing becomes comfortable.
クローディア - 恋人よ （五輪真弓） Also as a final note, I leave you with Zarah Leander - showing how low and strong the female voice can be!
