Here are three examples of my singing. The first two examples are sustainable. I can sing for hours on end and the next day I am ready to sing again. In fact, the first example is basically sight singing and was recorded at the end of hours of practice. The voice is as fresh as it was at the beginning of the day. The third example is unsustainable. It leaves me hoarse.
The difference is that the first two examples are an extension of my speaking voice. The third example is not. By the way, I wrote the Māori waiata (song), I hope you like it.