Singing in a dream often points to authenticity -- your own voice, literally and figuratively, being given room to come through. Singing confidently in front of others can reflect a readiness to be heard, while losing your voice mid-song often names a fear that what you have to say won't land the way you want.
The Scribe should be asked for manifestation of the hidden, study of mysteries, unfastening of the tongue, circulation of wealth, defamation and transmission of slanders. - Picatrix (Andalusia, ~1000.AD)