I admire your devotion and faith but one principle of Catholic teaching is that grace builds on nature. One effect of this is that it's only in very, very rare circumstances that God grants instantaneous transformations and it's almost always on His terms, not ours. I would not expect a miraculous singing voice after forty days of fasting, assuming you survive, which is an open question.
Here is my recommendation. As I said, grace builds on nature. Another consequence of this is that God usually prefers to work through ordinary, non-miraculous means. If you had cancer, you would not stay home and fast for forty days to be miraculously healed. You'd go to the doctor and pray.
I'm going to make the same recommendation to you. Find a voice teacher. Voices can be developed naturally. See how that works. Meanwhile pray fervently to God and do a novena — nine consecutive days of prayer — to St. Cecilia, the patron saint of singers and other musicians.
Her feast day is coming up on November 22nd. This is much more reasonable and more traditional than fasting, especially if you had intended to do an absolute fast with no food. Depending on your weight and health that could be deadly. People have died in less than 40 days of absolute fasting.
If you are dead set on fasting, and your doctor and spiritual director approve, consider a mitigated fast of one ordinary (i.e. not reduced), balanced meal a day and under no circumstances fast from water.