Methods in Singing Synthesis
Well, there is sampling of singing voices.  it's not the latest.  one thing that you perhaps need to worry about is that, unless you get a sample root for each note going up the MIDI note scale, then you will likely be pitch shifting the notes up or down a little.  it turns out that this causes a noticeable distortion sometimes called "munchkinization" or the "Alvin the Chipmunk" effect.  to avoid that, each cycle of the quasi-periodic note needs to be isolated into little "wavelets" (not to be confused by the wavelet transform) or "grains" or "FOFs".  then you would output the grains or FOFs at a rate of the fundamental frequency of the note without stretching or scrunching the grain (this requires overlapping and adding).  there's an old paper by Keith Lent that discusses pitch shifting vocal notes without shifting formants, and another paper by me that analyzes the algorithm a little more rigorously.  the latter, i can send a copy.

but for synthesis, you need not perform the real-time pitch detection that you would have to for pitch-shifting the voice live.  what you can do is sample some notes from these sung voices and perform analysis on them off-line and generate a file of these grains or FOFs and store them in your synthesizer.