In my early forties I worked in a job where some of the employees had a jam session after work one night a week. I became aware of it while working back one night. After that I stayed back regularly to listen.
It was led by a guy whose enthusiasm exceeded his talent but he was having such fun it was a very safe and encouraging environment for people to participate, especially after a beer or two.
One night they were performing Del Shannon's "Runaway". Nobody could manage to sing the wah-wah-wah-wah in the chorus so they just skipped it. After the second skip I thought to myself, "I can do that" and just dropped in at the next round. The other guys nearly fell over in surprise. (While working from home in my previous job I always played music, mostly female vocalists, so my high register and falsetto were quite good.)
Over the next couple weeks I discovered I was actually quite a reasonable vocalist although I had no musical training. My skills developed quite quickly and I found I really enjoyed singing harmonies even though I had no real understanding of musical theory. (I put that down to singing to music by TLC a lot.)
One night I noticed a beat between my voice and a guitar, suggesting I was off key, yet I felt sure I wasn't. I decided I needed to research this and discovered what was going on. I came to understand the concept of different temperaments and that, like all Equal Tempered instruments, it was actually the guitar that was slightly flat on the Fifth. The beat was there because I had apparently been singing the Perfect Fifth in my harmony. I realised why guitarists often bend notes. It stretches the string, lifting its frequency so it sounds the Perfect Fifth.
Armed with so much more information from a single night of reading about musical theory, I began applying my knowledge of resonances in electrical circuits and considering the sonic equivalents of same techniques used in tuning radio frequency circuits (particularly the Phase Locked Loop). The next jam session my vocal skills reached a whole new level even though I had done no practice on this new idea in between, something I found quite astounding.
My voice began tracking the guitars as they bent the notes. One of the guitarists told me he thought it was quite a remarkable skill. Another guitar I found I could really lock onto and resonate it to my voice. One night the guy playing it commented that he had to move away because "I was playing his guitar more than he was".
Vibrato just appeared in my voice at the right times without me thinking about it. (Definitely not Judith Durham style.) I realised this happens technique produces both the Equal Tempered and Perfect Fifths.
I decided to build a spreadsheet to study the differences between various tuning temperaments. I began to think there could be other scales with different temperaments and many more notes. I plugged these relationships into my spreadsheet and came up with a 43 pitch scale. To my amazement I discovered such a scale had already been invented long ago but never popularised. (It has weaknesses).
I eventually found my low register while recovering from a cold. After working on it I was able to get down very low. I studied the structures of the vocal tract and this also helped improve my voice so I could sing with less fatigue.
I came up with exercises to avoid gaps in my range and maintain resonance right across it. (One particularly useful one is to walk between rooms of very different acoustic qualities and try to maintain the sound quality and resonance as you pass through the doorway.)
Then I got a mic and amp and really broke loose.
Without any formal training and very little reading, learning through my own techniques, I eventually was able to sing nearly anything as lead, harmony or sub-harmony across a range of several octaves. I've lost some of that ability as I've gotten older (I'm 60 now) and not singing as much as I used to, but I still sing quite well when I do.