What is it that you mean by singing spells?
Spells that improve your singing (fake)
Spells that may give you more confidence while singing (possible)
Spells that give you a new singing voice (fake)
Spells that you cast by singing (possible)
You see there are so many different options.
I agree with the said post above.
If it's something that's trying to improve your singing then it most of the time won't work. But it's something like improving your confidecene etc, then maybe. It will still have a low chance of working. In my opinion.
Pretty much what H2O said.
I would argue that spells which improve singing are possible, but they must be coupled with practice and lessons (just like any spell). Think of something creative.